I haven't had a chance to check this myself yet.

For you, it worked in Firefox 85 but not in Firefox 86?

Firefox 86 introduced new cookie isolation features. I wonder whether those might be causing the issue? https://blog.mozilla.org/security/2021/02/23/total-cookie-protection/

Normally the best way to track down the exact patch that causes a new program is a tool called Mozregression. It is somewhat time-consuming (and bandwidth-consuming) to run, but hopefully at some point someone can use it to see what exactly broke it. https://mozilla.github.io/mozregression/

Thanks! I admit that I did not check which version it didn't work for. I was able to use Sharepoint in Firefox until around November/December of this year. It's been most of 2021 where I've had an issue.

Hmm, okay, Firefox 85 added a different kind of site isolation, so perhaps that's when the problem began? https://blog.mozilla.org/security/2021/01/26/supercookie-protections/

If we have to roll back to Firefox 84 or 83 or 82, it's hard for me to remember now what changes there were that might have been significant for cross-site authentication.