They try very hard to stop you trying, though; Microsoft Teams has removed the “continue without the Teams app” button entirely on Firefox, even though I'm fairly sure it still works fine in Firefox.
I'm assuming you're not on macOS, right? Even though Firefox supports H.264, it's not really consistent outside of macOS because Firefox doesn't want to deal with patent hassles. This is true of Slack as well, some developers would actually support if Firefox has actually have decent support, but it's inconsistent outside macOS, where it can rely on Apple's good integration.
P.S. OpeH264 doesn't help mainly because it's decoding bugs, not encoding bugs.
