I don't know about everyone, but one of the reasons I've stayed with FF over the years is that if they decided to change the UI, I could change it back through their customisation features (most of the time - I try to avoid third party add-ons where possible because they can result in the program malfunctioning). However, with changes in FF 26 and 28 (re: 28 - judging by what they've done with 26), they're moving away from that and copying Chrome and enforcing those changes. I don't like how Chrome looks and works, I don't think it is intuitive.
I said I'd go with Seamonkey, but either it can't do what I want in terms of cookie storage, or it's not that apparent, but I've completed changing over to Pale Moon (though admittedly I'm worried about the fact that Pale Moon says that it is version 24).
While I acknowledge that people don't like change 'full stop', I think such people (including myself), they perceive it as "change for the sake of change" (and sometimes their reaction might be "dislike for the sake of dislike of change"), like the morons who wrote things like "Apple has to change the look of iOS in order to "keep up"... why? It looked fine, they've changed the colour scheme now... great.), and UI changes are rarely *necessary*.
In the case of the newer download manager that FF 26 now forces the use of, why do I have to start either using keyboard shortcuts or clicking in two different places to bring up the downloads window when it appeared automatically before, I don't perceive this as an improvement, and someone would have to try quite hard to convince me that it would actually help (even in "the long run") when it is actually hindering my productivity.
When software makers forget how to innovate, they copy competitors instead. It's just moronic.
IMO, the UI should only change (or come up with a system for opting into a UI change without having to avoid security updates), when it actually results in a productivity improvement, not because of some fashion-season-thing that serves no useful purpose as far as the end user is concerned.