https://support.mozilla.org/en-US/kb/websites-say-cookies-are-blocked-unblock-them

Make sure you are not blocking content.

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

Did that. I tried all the obvious stuff before posting the problem.

REPEAT: It worked on Monday. Update installed on Tuesday. Today, it's broken the same way on three systems where it used to work. I'm pretty sure the update broke it.

There is a change in Firefox 96 related to cookies and insecure sites.

• https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/96#http

You can check if you have old bookmarks with the the http:// protocol that need to be fixed.

See also:

• /questions/1363998 blocked 3rd party cookies

As a workaround for now you can modify this pref on the about:config page to revert this change.

• about:config => network.cookie.sameSite.laxByDefault = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox