It "works", but requires far too much manual intervention, with frequent authentication popups, as opposed to just using Chrome/Edge and be done with it.
I'm not ruling out that some of it is due to service misconfiguration by the auth team and service teams, but configuration errors or not, the end user result is the same, Chrome or Edge simply offer the path of least resistance.
Name and shame I say.
I have had an issue with Linux and Microsoft SSO. Though that was more down to IT policies in Azure AD than it was Microsoft SSO specifically.
The only time Firefox was an issue with Microsoft SSO was when I tried to buy Minecraft and something about the Xbox / Minecraft / MS SSO required Edge. But that was one instance out of hundreds of interactions with MS SSO
At some point FF wasn't able to do the user verification under linux (ask for a pin). Even though that's been fixed now, azuread still throws an error.
Chromium on Linux works fine. I haven't tried FF on Windows.