There was no excuse, they should just bring xul back
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.
Ideally the functionality once implemented as monkey patching can be provided by a defined interface with the addon system and indeed this is exactly what one sees today with most broadly useful extensions ported.
Incidentally if you want to analogize Firefox vs Linux the interface that is not supposed to break is the interaction with the web itself to the degree that this is possible. Linux actually makes external out of tree addons which touch let alone modify the kernel break/update themselves constantly. Linus would never have been stupid enough to commit to something like XUL for the kernel wherein he himself would be responsible for keeping thousands of monkey patching addons working while trying to evolve the kernel.