Basically anything that relies on Microsoft SSO, which is pretty much everything "enterprise".
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.
I've almost never had an issue with Microsoft SSO in Firefox. Be that with corporate Azure AD or personal Microsoft accounts.
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
AzureAD doesn't work with Firefox on Linux if you need a yubikey or similar as a second factor.
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.
About 80% of the “for your consideration” sites I get links to each year in awards season don’t work on Firefox, they outright refuse to use anything but Chrome, Edge, or Safari. No doubt something to do with the fun stuff they do to visually watermark the stream such that they can prosecute my ass if I share it anywhere. My guess is loads of enterprise-internal websites also just refuse to support more than one blessed browser.
I use Firefox only, and use it for work to open outlook apps as well. I'm on Linux. Last week though I was traveling so my setup was my wife's macbook, with Firefox. Outlook would hang at the password login, but it worked fine on Safari. No idea why. I tried multiple times, tried emptying my cache and deleting internwt files, nothing... Anyone else ever had this issue?
The only issue I've seen is, that some apps -- Proxmox, Jitsi Meet (via jitsi-openid) -- fail to authenticate against Keycloak. Sometimes. Sometimes clearing cookies helps; sometimes not.
But the weird thing is that this failure happens only on one computer (running MacOS). I have another, with synced settings & extensions & ublock settings (running Linux) and there it always works?
For a normal, public site, I've not seen any issue.
There are definitely a few javascript differences from Chrome and Firefox. Just look at the landing page for caniuse.com to see there are definite things that are not available in FF.
Still nobody can actually link a single site which doesnt work in Firefox.
I have been using it nearly 20 years, and still never seen a site I couldnt use in Firefox.
Please somebody prove me wrong so that I dont just think everyone that says this is just doing something wrong!
EDIT: Not sure why I am getting downvoted, am I lying? If someone posts a link that doesnt work on FF I would be happy to concede. But I have been asking this for decades and nobody has ever given me one.
I have zero problems with that site using Firefox on Linux, Android, or Windows.
Curiously, most complaints seem to be from Mac users, and not just for this site, leading me to believe that something different may be going on on MacOS builds or APIs that it has to use there.
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.