People wanted a faster, more secure, multi-threaded experience.

> Note: Having read in comments that some users apparently do not care about security, let me add that being secure is a really, really important point for Mozilla and has been since the first day. Regardless of add-ons, not having security means that an exploit is eventually going to show up that will steal user’s passwords and use them to steal their bank accounts – and that exploit will get sold around and will soon show up everywhere. Firefox developers fight this threat daily by all sorts of means, including code reviews, defensive programming, crash scene investigations, several types of sandboxing, static analysis, memory-safe languages, … Consequently, for Mozilla, if a feature prevents us from achieving great security, we always pick security over features.

You should read the link.