This error code usually means that the server response was not what was expected. Could you try:

(1) Prevent Firefox from using any system-level proxy

Open the Settings page using either:

• "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• type or paste about:preferences into the address bar and press Enter to load it

In the very tiny search box at the top of the page, type proxy and Firefox should filter to the "Network Settings" section.

Click the Settings button, select "No Proxy" near the top of the dialog, and click OK near the bottom of the dialog.

Any difference?

(2) Clear Firefox's Cache

See: How to clear the Firefox cache (only select Cached web content, don't clear all cookies and site data)

If you have a large hard drive, this might take a few minutes.

(3) Remove your Google cookies (save any pending work first). While viewing a page on Google search, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page. Can you get into Gmail now?

You can try to disable DNS over HTTPS.

• https://support.mozilla.org/en-US/kb/firefox-dns-over-https
• https://support.mozilla.org/en-US/kb/dns-over-https-doh-faqs

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors
• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
• https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

See also:

• https://appuals.com/pr-end-of-file-error/