Firefox has been my goto browser for years but recently I've noticed it screwing things up, most notable when viewing source code in github. The text highlighting gets broken somehow.

Then it’s GitHub fault, because hihglighting works everywhere else. When has it becomes the default to blame the platform because some program have bad code?

Yes, for me as well, but once a year I will encounter some web page thst won"t do what it is meant to unless you use chrome.

In half of those cases, changing your user agent to Chrome magically solves all the issues.

This is literally some browsers' solution. They have an internal domain list and use the chrome UA for those