I have no good answer for you. Possibly if this happened before the answer then will work. Google search does not turn up anything current https://www.google.ca/search?q=firefox+error+MOZILLA_PKIX_ERROR_INVALID_INTEGER_ENCODING&ie=utf-8&oe=utf-8&gws_rd=cr&ei=tFx5WaqOLMqh0wKXh6aYBg

Suggest https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode if that fails to help follow the refresh information. If that fails uninstall and delete folders and download a full version installer https://www.mozilla.org/en-US/firefox/all/ or wait till on or about Aug 8th for version 55 release and see if that fixes the issue.

Please let us know if this solved your issue or if need further assistance.

Gmail is a pretty heavily used site. If this was a standard condition, I think there would have been more reports by now.

Have you already tried "the usual"? If a site is generally known to work in Firefox, these are general suggestions to try when it stops working:

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button

In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog so you can remove that site's cookies individually.

Then try reloading the page. Does that help?

Testing in Firefox's Safe Mode: In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)