This is probably due to DRM which is poorly supported by Linux (lazy devs will probably just exile Linux completely). But I do remember some linux-only decoder issues in the past. Decoder and codec support is just poorly signaled overall, even if an API says it supports it, it may not support a specific profile or even support it well.

Some of our video test grid machines are Ubuntu, so from our end Twitch video should work pretty well on Linux :-)

This was so common in Opera 12 that there was a dedicated button in the UI to change the user agent for a page.

The browser also included a built-in user agent compatibility list and the user could manually add sites to use a specific UA string.

Most sites would work fine in Opera with the correct UA. Many did not work when using the default UA with the word "Opera" in it.

Firefox has a built-in list of websites that need UA tweaks to work in Firefox. (In fact, Firefox's UA override feature was designed by ex-Opera engineers at Mozilla.) You can review the list (and toggle them) in Firefox's about:compat page.

A recent success story: Mozilla recently added a UA tweak to work around Slack's Chrome-only check for video calls. Slack engineers reached out to Mozilla and, just a few days later, Slack removed their Chrome check and now support video calls in Firefox without a UA tweak. :)

Except if your UA reports Firefox on Linux, where calls and huddles remain unavailable to this day.

I wouldn't call that a success.

A very curious case is that huddles for me work in wayland (under sway), and don't work in X11 (under i3), for the exact same configuration otherwise. I have not investigated what exactly the browser sends.

Do the calls and huddles work in Chrome on Linux? Do they work in Firefox if you use a UA that doesn't mention Linux?

Yes and yes.

Thanks for testing. I'll ping Mozilla's web compat team about this Slack Linux problem. (I work at Mozilla.)

I was told Slack has some Linux-specific issues that they're working on with the Firefox team, such as some complications around screen sharing depending on the window manager (as mentioned in some other comments below about Wayland vs X11).

I couldn't use those through Safari on a Mac either. Nothing really tells you that it's unsupported. I could search or figure it out but I don't wear my programmer/software tester hat all the time.

Welp, better spend a few minutes downloading the same thing but in a Chrome sandbox.

Can't say I've seen that often, but there was a short period of time when Twitch refused to play on a FreeBSD user-agent. Spoofing as Linux or Windows worked. They did fix it.

I really not have the need to do that. Perhaps on a few weird sites. But it's far from being the usual thing to see a video on a browser.