I can't login to the work-paid-for version of Microsoft Copilot with Firefox, for some reason. I've had one or two others - I think they were internal CMU website tools. And even more niche: My kids took a ski lesson last year at Snowbird and the website with their report card rejected anything that didn't identify as Chrome. It _worked_ with mobile FF, but it popped up a "YOU SHOULD USE CHROME" banner and wouldn't let me past.

So, small stuff. Maybe Copilot isn't working because of ublock, though.

You can also disable ublock on specific 'trusted' domains.

In my experience if something isn't working in firefox, it's more likely that it's firefox antitracking setting much more often than ublock. You might try turning that off first.

I've been daily driving Firefox for several years. Everything I use on a daily basis works fine on FF, but every now and then you come across some random site that doesn't load or loads poorly.

The Teams web app doesn't work (I refuse to install the OS level app)

Why? I can just temporarily launch Chrome if I need to join a meeting. There's no need to have MSware running in the background 24/7 doing god knows what.

Expedia doesn't render properly in Firefox - some of the sections are missing, but it's not immediately obvious what is missing. It took me a while to figure out why my wife kept having problems with that site, and I had to move her to Chrome to allow her to use it.

I continue to use Firefox because I know when to suspect a website problem might be the browser, but she doesn't have the ability to analyze a situation like this. I have this conundrum with other family members that I support. I want them to use Firefox, but I hate to have them run into an issue because of the browser I recommended.

Do you have Enhanced Tracking Protection on? It often blocks this kind of third party content

I always turn off firefox's blockers and just use ublock origin with a few more lists like the annoyance and cookie lists

Bizzarely, Microsoft Word on web seems to be the only thing I've encountered which has FireFox problems, specifically it'll periodically "save" the document I'm typing and then delete the last sentence or two that I changed while it was saving. I think it's some kind of broken state management on the MS side (leave it to MS). That's the only site I've used though, and I've been a daily driver of FF for 10 years.

Sticky desktop notifications don’t work on macOS on FF, which means event notifications from calendar apps disappear within a few seconds, leaving no time to snooze them or act on them in any other way. Queue missed meetings.

Amazon properties (the storefront, Prime) have been quite flaky for a long time now, but that may just be me.

Certainly no major site.

I subscribed to SlingTV a little over a year ago and it did not support Firefox, even with all the DRM enabled. Although that's a problem I blame on SlingTV, not Firefox. It was a known issue which they refused to address. I've since ended my subscription with them.

IIRC, there was also a time when Netflix did not support its highest streaming quality on Firefox. I'm not sure if that's still the case since I also ended my Netflix subscription.

Otherwise I cannot think of any major site which is not supported on Firefox. Outside my employer's fragile intranet, I can't think of any sites which do not support Firefox.

with firefox I had trouble with netflix and youtube. These two sites worked much smoother and consistent on chrome.

I can't get ticketmaster to work reliably on Firefox. I guess it thinks I'm a bot. I can use Chrome on the same computer and book tickets just fine.

One crap product forcing me to use another crap product! ;)