The destruction of it's ecosystem.
Too many choices have been made to simplify Firefox when maybe they should have done a bit more spelunking to see what the users were actually using.
Taking away the status bar. Yeah, there are multiple extensions to get that back, the trouble being that they aren't the original status bar and some of the extensions that I use expect the old status bar, not the extension status bar. Update that extension? Well, the person writing that extension has thrown in the towel. When other issues cropped up, somebody else did come along and fix the issues, but the original programmer can come around and kill it because it's still technically his copyright. Yeah, he didn't GPL or put any other kind of license on it. So, it might exist today, but tomorrow it won't.
Making Firefox look like Chrome is just stupid in my book. There was zero reason to change it. Talk about getting the desktop to look like the mobile is pure crap. They are different environments. What works on a phone or tablet doesn't necessarily mean that it works on the desktop, even Microsoft has figured that part out with Windows 10 coming out now. Extremely obvious to me, so I must be a genius. Or not.
They have changed things such that old themes no longer work. The old personas, which I guess are now considered to be theme extensions, seem to be the only new themes actually getting developed. And they're ugly.
Their mobile push (for Firefox OS) was interesting, but again, desktop seemed to suffer again because of it. They started actually pushing a 64-bit version of Firefox on their Nightly page. Then decided that tracking those bugs specific to it might be too much, so they decided to stop it, then after an outcry, decided to keep doing the 64-bit builds, but if you had a problem, don't bother filling a bug for it unless it also happened on the 32-bit version. And then they decided to back track on that as well. You just can't find the 64-bit version on the Nightly page anymore. But it can be found, at least.
I run the 64 and 32 bit Nightlies, release and beta versions. And they work for me. At least for now.
I don't like IE. Chrome works. I'm just not sure I want Google tracking me that much.