The one that got me was when they took away 'n' to get to the next search result. Not anything to rage quit over, but it pissed me off. I now use VimFX for a few vim navigation keys, and vim-like search.
It wasn't the n for vim search I was missing, it was the next function with a keypress, which happened to have been done with n in FF's searchbox, and coincidentally n in vim. I didn't know that they replaced it with F3.