If you use Firefox from the repositories then you can try Firefox from the Mozilla server to see if that makes a difference. You can check for issues with Total Cookie Protection. *https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq *https://support.mozilla.org/en-US/kb/introducing-total-cookie-protection-standard-mode See also: *https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords You can check the Web Console (Tools -> Web Developer) for messages about blocked content and about the presence of mixed or unsafe content. *https://firefox-source-docs.mozilla.org/devtools-user/web_console/ You can check the Network Monitor to see if content is blocked or otherwise fails to load. *https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/ If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.

I was getting 403 forbidden in the log. I downgraded to 108 to test and it forced a new profile and worked. Upgraded to 110 and it worked with the new profile, but not the old profile. No idea what is wrong in the other profile. I added back most of my addons and logged back into sync. Really not sure why I'd be getting a 403 with one profile and not the other.