You need to sign, fill out, or mark up a PDF, and the Firefox PDF editor already built into your browser can do it with nothing extra to install. It adds annotations on top of the file. It doesn’t rewrite existing text, and its signature and cover-up tools aren’t a substitute for a verified e-signature or true redaction, so always save a separate copy and reopen it to confirm your edits stuck.
What Is the Firefox PDF Editor?
Firefox has a built-in PDF viewer, and that viewer includes a lightweight editing toolbar. When you open a PDF in a Firefox tab, tools for filling forms, adding text, highlighting, drawing, inserting images, commenting, and adding a signature appear across the top of the document (Firefox PDF Editor feature page). It’s free, and it needs no account or add-on.
Before you rely on it, know where its limits are:
| You want to… | Can Firefox do it? | What actually happens |
|---|---|---|
| Fill in an interactive form | Yes | You type into the form’s own fields |
| Change words already in the PDF | No | The Text tool only adds new text on top of the page |
| Sign a document | Partly | You add a typed, drawn, or image signature annotation placed on top of the page. Confirm with the recipient whether this meets their requirements before treating it as a binding signature |
| Hide sensitive info | No | A drawn box covers text visually, but it isn’t a dedicated redaction feature, so don’t rely on it to permanently remove sensitive information |
| Reorder or delete pages | Yes (Firefox 150 and later) | Firefox saves your changes to a new PDF rather than editing in place (Firefox 150.0 release notes) |
Firefox’s signature tool is built for convenience, not legal certainty. For contracts, leases, or anything that needs a legally binding, identity-verified signature, use a dedicated e-signature service such as Adobe Acrobat Sign or DocuSign.
Before You Begin
- Desktop Firefox (Release channel) — this guide was tested on Windows and macOS
- A PDF file saved on your computer (if it came as an email attachment, download it first instead of using the preview)
- Optional: an image file of your handwritten signature, if you want to use the Image signature option
- Optional: a touchscreen laptop, stylus, or drawing tablet for neater drawn signatures
| Requirement | Details |
|---|---|
| Device | Windows PC or Mac (other desktop platforms weren’t verified for this guide) |
| Browser | Firefox Release channel, kept up to date (release notes) |
| Account | None required |
| Mobile | Not covered. Android and iOS Firefox feature parity wasn’t verified |
Step-by-Step Guide
Step 1: Update Firefox
Page tools and other editor features arrived in specific versions, so start current.
- Open Firefox.
- Click the menu button (three horizontal lines) in the top-right corner.
- Go to Help > About Firefox.
- Let Firefox check for and download any update. Click Restart to update Firefox if it appears.
Expected result: The About window shows “Firefox is up to date” and lists your version number.
On a Mac, the steps are identical. You can also reach the same window from the menu bar via Firefox > About Firefox.
Step 2: Set PDFs to Open in Firefox
If this setting points somewhere else, PDFs download or open in another app and you never see the editing toolbar.
- Click the menu button and choose Settings.
- Stay on the General panel and scroll down to Applications.
- Find the Portable Document Format (PDF) row.
- Click the dropdown in the Action column and choose Open in Firefox.
Expected result: The PDF row now reads Open in Firefox. The change saves automatically.
Tip: You can paste
about:preferences#generalinto the address bar to jump straight to this page. Settings labels can shift between releases, so if you don’t see Applications, type
Step 3: Open Your Local PDF
On Windows
- Open File Explorer and find your PDF.
- Drag the file onto an open Firefox tab or onto the tab strip.
- Alternatively, right-click the PDF and choose Open with > Firefox.
You can also use File > Open File inside Firefox, browse to the file, and click Open.
On macOS
- Open Finder and locate your PDF.
- Drag it onto a Firefox tab, or Control-click the file and choose Open With > Firefox.
In Firefox, you can also use File > Open File from the menu bar.
Expected result: The PDF loads in a Firefox tab, with the editing toolbar in the top-right area of the viewer.
Step 4: Fill Out Form Fields
Interactive PDFs, like many applications and intake sheets, have built-in fields.
- Click inside a text field and type your entry.
- Click checkboxes or radio buttons to select them.
- Press
Tabto move to the next field.
Expected result: Your text appears inside the form’s own boxes. If clicking does nothing, the PDF has no fillable fields. Use the Text tool in Step 5.
Step 5: Add Text Where There’s No Field
- Click the Text tool in the editing toolbar.
- Click the spot on the page where you want text.
- Type your text, then pick a color from the tool’s options.
- Click outside the box to finish. Drag the box later if it needs moving.
Warning: The Text tool places new text on top of the page. It can’t change or delete the PDF’s existing words. If the original text must be rewritten, you need a full PDF editing application.
Step 6: Highlight Important Passages
- Click the Highlight tool.
- Drag across the text or area you want to mark.
- Pick a highlight color.
To change a highlight later, click it to recolor or delete it.
Step 7: Draw on the Page
- Click the Draw tool.
- Set the color, thickness, and opacity.
- Draw with your mouse, trackpad, touchscreen, or stylus.
This is handy for circling a figure or sketching an arrow.
Warning: Don’t rely on the Draw or Highlight tool to black out a Social Security number, account number, or other sensitive data. These tools cover content visually but aren’t a dedicated redaction feature. Use a tool built for secure redaction if you need to permanently remove sensitive content.
Step 8: Insert an Image
- Click the Image tool.
- Choose an image file from your computer.
- Drag the corners to resize it, then drag the image into position.
- Add alt text when prompted so screen reader users know what the image shows.
Step 9: Leave a Comment
- Select the text or area you want to comment on.
- Click the Comment tool and type your note.
- To review every comment in a long document, open the comments menu in the top-right of the PDF toolbar. A sidebar lists each comment, where you can read or edit it.
Step 10: Add a Signature
- Click the Signature tool and choose Add new signature.
- Pick a tab:
- Type: Type your name, and Firefox renders it as a signature.
- Draw: Sign with your mouse, trackpad, or stylus.
- Image: Upload a picture or scan of your handwritten signature.
- Check whether Save signature is selected. Leave it checked to reuse the signature later, or uncheck it on a shared computer.
- Click Add.
- Drag the signature into place and resize it.
Expected result: Your signature appears on the page as a movable annotation.
Warning: This signature is added as an annotation on the page, not a cryptographically verified signature. It works well for an informal sign-off, like a permission slip or an internal form. If someone asks for a “verified” or “legally binding” e-signature, use a dedicated e-signature service instead.
A clean scan from a document scanner or a photo of your signature on plain white paper usually makes the neatest Image signature. For a drawn signature, a Wacom drawing tablet or a stylus beats a trackpad.
Step 11: Save Your Work as a New Copy
Nothing you’ve done is saved yet. Your edits live only in that tab until you download a copy.
- Click the Download (save) button in the PDF toolbar.
- If Firefox asks where to save, pick a folder and give the file a new name, such as
form-signed.pdf. - If Firefox doesn’t ask, the file goes to your Downloads folder.
On a Mac, the button is in the same place. The save dialog is a standard Finder sheet.
Expected result: A new PDF containing your annotations is saved as a separate file. The file you originally opened isn’t edited in place.
Step 12: Reopen the Saved Copy to Verify
This is the step most people skip, and it’s how you catch lost edits before you email the file.
- Close the Firefox tab with your edits.
- Open the new file you just saved (repeat Step 3).
- Check that every field, highlight, comment, and signature is there.
- For extra confidence, open the copy in a second PDF app, such as Adobe Acrobat Reader or Edge on Windows, or Preview on a Mac.
Expected result: Everything you added shows up. If something is missing, go back to the edited tab (if it’s still open) and save again.
Configuration
These are the settings worth knowing about:
- PDF handling (Settings > General > Applications > Portable Document Format (PDF), though exact labels can vary by version): This controls whether PDFs open in Firefox, prompt you, or go to another app. The default varies by installation, so check it rather than assuming.
- Save signature (signature dialog): This keeps your signature in Firefox for reuse. Confirm the checkbox state each time before adding a signature, especially on a shared PC, since the default can vary.
- Download location (Settings > General > Downloads): Turn on Always ask you where to save files if you want to name each annotated copy yourself instead of sending it to Downloads.
Tips and Troubleshooting
Fix #1: Check the PDF handling setting
If a PDF downloads instead of opening, or opens in another app, the setting from Step 2 is the usual cause.
- Go to Settings > General > Applications.
- Set Portable Document Format (PDF) to Open in Firefox.
- Reload the PDF link, or reopen the file.
Fix #2: Download the file, then open it locally
Some websites send PDFs with a misconfigured file type, and Firefox may not display those in the viewer (Mozilla Support).
- Save the PDF to your computer.
- Drag the saved file into a Firefox tab, as in Step 3.
Fix #3: Update Firefox for missing tools
If page reordering or another tool isn’t there, check your version.
- Go to Help > About Firefox.
- Install any pending update and restart.
- Confirm you’re on the Release channel. Page operations require Firefox 150 or later.
Fix #4: Use another PDF app
If one stubborn file still won’t open in Firefox, open it in your system’s default PDF app or Adobe Acrobat Reader instead. A file that fails in every app is probably damaged, so ask the sender for a fresh copy.
Quick answers
- My annotations vanished. You closed the tab before downloading. Edits are never written back to the original file. Always do Steps 11 and 12.
- I can’t change a typo in the original text. The Firefox PDF editor only adds content on top of the page. Add a correction with the Text tool, or use a full PDF editor.
- Comments are hard to find. Open the comments sidebar from the top-right of the toolbar.
- Do I need an extension? No. Everything here is built in.
Wrapping Up
For form filling, contract highlighting, or a quick sign-off, Firefox’s built-in editor is honestly all most people need, and the download-then-reopen habit is what keeps your work from disappearing. Just don’t push it past its limits. If the document needs a verified signature or real redaction, switch to a proper e-signature or redaction tool. If PDFs from one particular site never open, the problem is usually that site’s file setup, not your PC.
| Step | Action | Applies To |
|---|---|---|
| 1–2 | Update Firefox, set PDFs to Open in Firefox | Windows, macOS |
| 3–10 | Open the PDF, then fill, annotate, comment, sign | Windows, macOS |
| 11–12 | Download a new copy, reopen it to verify | Windows, macOS |