People always say this but almost never are they able to give examples.
The web interfaces for both Jitsi and Zoom fail to recognise my web camera. I've gone through the permissions settings a dozen times and Firefox says that it sees camera, but it only displays a black feed. Vivaldi correctly accesses the camera on both sites.
imascientist.org.uk makes it "impossible" to sign up for outreach sessions under Firefox. The div with the sessions has a `max-width` parameter, but the contents are significantly larger than this width, resulting in the buttons being outside the rendering area and inaccessible. I have to go into the dev tools and edit the CSS every time I open the page.
These are just the issues that I've dealt with today. Don't get me wrong - I'm literally typing this in Firefox at this moment. However, it doesn't help anyone to pretend that these issues never pop up. I simply feel that the benefits outweight these annoyances.
zoom worked without problem.. both audio and video..
imascientist.org.uk i opened the registration page and it showed without problem buttons where in the end of the page as i would expect.
It look like it is something in your end and not with firefox..
On imascientist.org.uk I don't see a sign up button, only login.
When you have a site not working as expected, start Firefox in "Troubleshoot Mode" (alt-key then in the menu at the top Help > Troubleshot mode) that starts Firefox with addons turn off. If the site works, restart and turn of addons one by one to you see which one creates the problem.
The "give me examples" is sort of a moving target, anecdotally I have run into many more completely or partially broken sites when using Firefox as compared to Chrome and Safari. It is likely that some or all of those examples are now fixed but the pain in the moment is enough to keep me from using the browser long term.
It works fine for me.