I use GitHub frequently and have never came across that issue; can't speak for the other sites though. If you're sure it's not the website's fault I'd encourage you to submit feedback: https://webcompat.com/issues/new
There's an open issue about it, so hopefully github should fix it eventually. It's on firefox mobile
Firefox mobile is its own beast. I experience a whole host of issues on it, but have never had issues with Firefox on desktop. Only reason I use FF for Android is for uBlock, but it's so unreliable I'm dying for a better option...
These days desktop and mobile is kinda joined at the hip because users expect things like tabs and history to sync. So if Firefox has many problems on mobile to the point where users are actively deterred from using it, that hurts Firefox on the desktop as well.