Most Googlers use Chrome at work so it’s going to be better supported incidentally. The reason behaviors change when switching UA is because features vary across browsers and the UA ends up acting as a flag that turns features on or off. Not because someone is putting in a sleep() when they see Firefox. That would make for a very awkward code review.
When I was a Googler I exclusively used Firefox. Things were never so bad that I had to switch over to Chrome even with internal tools, though I suffered many hiccups due to this choice.