Downvotes? Someone doesn't like facts? OK:
* XUL abandoned and with it thousands of powerful extensions (a ton of them have never been reimplemented)
* UI has been changed great many times with the last iteration probably the most horrible one (considering all the white space, huge fonts, gray nondescript icons) - on Reddit people were really unhappy
* Features removed
* Full themes support removed
* Browser customization shrunk
* Only a couple of years ago Firefox stopped leaking RAM like crazy
And here's their last feat:
https://news.ycombinator.com/item?id=30375640
Yeah, it's all "because of" Google Chrome. Really. Except Mozilla has actively been shooting themselves in the foot for the past decade.