I mostly use Firefox so making sure projects work with it happens naturally. Also, using a decent framework like Bootstrap for simple projects automatically makes code Firefox compatible.

But I'm more frequently noticing that sites don't work with Firefox, that I have to switch to Chrome. The worst issue so far has been using WordPress; the default post editor simply doesn't work reliably with Firefox when doing basic text editing. That means Firefox can't be used on millions on sites that run WordPress. That's just sad.