Fix File Attachment Uploads Failing in Firefox Webmail

You click the paperclip in Gmail or Outlook on the web, pick a file, and nothing happens. Firefox breaks the upload when the webmail domain is missing a permission, when its cookies or cached scripts have gone stale, or when a privacy extension strips the upload form.

Below are the desktop Firefox settings to check, in the order that finds the cause fastest, so attachments start working again.

In this guide, I have covered:

Why your webmail attachment won’t upload in Firefox

You click the paperclip in Gmail, Outlook on the web, or Yahoo Mail, choose a file, and nothing happens. Sometimes the picker closes and the upload stalls at zero. Firefox is the middleman in both cases: it hands the file to the page through the browser’s file picker, and the page pushes it out over your existing session.

That handoff breaks quietly. The webmail domain may be missing a permission it needs, a privacy extension may strip the upload form, or the saved session in Firefox’s cookies may have gone stale.

Nearly every fix lives in two places: per-site permissions for the webmail domain, and Firefox’s privacy controls. Work through them in the order below and you will usually land on the cause within a few minutes.

Quick checks before you change any settings

Rule out the boring causes first, so you don’t spend an hour on settings that were never the problem.

• Check the file size against your provider’s limit. Gmail caps at 25 MB, Outlook.com at about 33 MB, Yahoo Mail at 25 MB. Oversized files fail without a clear error.
• Check that the file name uses ordinary characters and a supported extension such as PDF, JPG, PNG, DOCX, or ZIP.
• Open the same webmail in Chrome, Edge, or Safari. If the upload works there, the cause is Firefox-specific and the rest of this guide applies.
• Fully close Firefox (File → Exit, or Ctrl+Q) and reopen it. A stuck upload can block the file picker until the browser restarts.
• Confirm Firefox is current (Help → About Firefox). Older builds occasionally hit upload bugs that later releases patch.

Allow the webmail site to use cookies and JavaScript

An upload needs two things from the page: a valid session cookie and working JavaScript for the upload widget. Block either one and the attach button may open the picker but never finish. The cleanest fix is to view site info and set site settings in Firefox for the webmail domain.

1. Open the webmail tab and click the lock (or info) icon at the left end of the address bar.
2. In the pop-up that opens, find the Permissions for this site section.
3. Set “Send and receive cookies” to Allow.
4. Set “Open pop-up windows” to Allow if your webmail opens the upload form in a pop-up.
5. Close the pop-up and reload the page with F5.
6. Try attaching a file again.

Some Firefox versions show the lock icon pop-up without a permissions list. The same per-site controls live in Settings → Privacy & Security → Permissions in that case.

Turn off Enhanced Tracking Protection for the webmail site

Upload widgets often pull resources from a different domain, such as an upload CDN or an identity provider. Enhanced Tracking Protection can block those requests, and the upload dies without a message. Allowing the one domain restores the normal upload path without weakening protection anywhere else.

1. Open the website that fails to load in Firefox.
2. Click the shield icon to the left of the address bar.
3. Toggle the switch labeled "Enhanced Tracking Protection is ON for this site" so it moves to OFF.
4. Wait for Firefox to reload the tab automatically, then try the page again.

Reload the webmail tab and attach the file again. You’ll know it worked when the progress bar moves and the attachment shows up in the message. If it does, keep protection off for that domain only.

Disable extensions that may block uploads

Ad blockers, privacy add-ons, and script blockers (uBlock Origin, Privacy Badger, NoScript, Ghostery, and similar) sometimes hide the attach button or break the upload widget. Disabling them temporarily is the fastest way to confirm one of them is behind it.

1. Click the hamburger menu and select Add-ons and themes.
2. In the left sidebar, click Extensions.
3. Use the blue toggle next to each extension to disable it. Start with ad blockers, tab managers, and any extension you installed in the past few weeks.
4. Restart Firefox from the hamburger menu .
5. Use Firefox normally for a few minutes, then check about:performance. If usage dropped sharply, re-enable extensions one by one to find the offender.

Reload the webmail tab once Firefox restarts without extensions, then try attaching. If the upload works, turn your extensions back on one at a time until the problem returns. The last one you enabled is the culprit, and it needs a site exception or a whitelist entry for your webmail domain.

Clear cookies and cache for the webmail domain

Attachments often stall after the file picker closes because of stale cookies or cached scripts. Clearing the site data forces the page to fetch fresh scripts and start a fresh session. If you would rather manage cookies one domain at a time, here is how to block or allow browser cookies in Firefox.

1. Open the menu at the top right of Firefox and click Settings.
2. Select Privacy & Security from the left sidebar.
3. Scroll to the Cookies and Site Data section and click Manage Data….
4. Type the site's name into the Search websites field to find it faster.
5. Select the site, then click Remove Selected.
6. Click Save Changes and confirm the prompt.
7. Reopen the site in a new tab and pin it again.

Reload the webmail tab, sign back in, and attach the file. The upload should now go through on a clean session.

If site data keeps coming back after you clear it, that is a separate problem. See Firefox not clearing site data on close for the setting that controls it.

Test the upload in a private window

A private window carries no stored cookies, no cached scripts, and no saved permissions. That makes it a clean test. Press Ctrl+Shift+P, sign into your webmail, and attach the same file. If the upload works there, the problem lives in your normal profile (extensions, cookies, or saved permissions) and the earlier sections will fix it.

A successful upload in a private window does not change your main profile. It only narrows the cause, so you still need to make the matching change in your normal Firefox setup.

Try Firefox Troubleshoot Mode

Troubleshoot Mode restarts Firefox with extensions, themes, and some custom settings turned off while keeping your data intact. Use it when you want a yes-or-no answer on whether the problem is Firefox-wide or limited to your profile.

1. Click the hamburger menu at the top right of Firefox.
2. Click Help.
3. Select Troubleshoot Mode….
4. Click Restart in the confirmation dialog.

If uploads work in Troubleshoot Mode, an extension or custom setting is the cause. Exit the mode and work through the extension steps above to find the trigger.

Update Firefox

Older Firefox versions occasionally hit upload bugs that later point releases patch, so confirm you are on the current stable build before digging further.

1. Open the Firefox browser app on your computer.
2. Click on the More menu and select the Settings menu.
   
3. Scroll down to the Firefox Updates section within the General tab, and click the Check for updates button.
   
4. Firefox will check for updates and download them. It may ask to restart to complete the installation of the update.
   The browser will be updated to the latest version.

Restart Firefox after the update, reload the webmail tab, and retry the attachment. The Firefox release notes list the most recent upload fixes.

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