In the last five years or so Firefox has increasingly introduced controversial changes that make it (IMHO) less good, primarily around interface design. And, from what I understand, Mozilla employs full-time UX designers who've been driving much of that. Of course, with Firefox it's still possible to modify, fix and restore all these recent interface "improvements" with user CSS but it's a constant annoyance to need to keep fixing it. Fortunately, there's an active community effort around restoring the Firefox interface and usability, exemplified by the brilliant Lepton project https://github.com/black7375/Firefox-UI-Fix/releases.
My perception just watching the evolution of Firefox from the outside, is that it used to be a browser that celebrated the ethos of "Have it Your Way." However, Mozilla the company gets money to pay its executives and employees (millions in the case of more than one recent CEO) by actively driving users and eyeballs for Google, Pocket and other advertisers. So the company is highly incentivized to try myriad changes and redesigns to increase appeal to "the masses" of browser users. Thus, the UX keeps getting 'simplified' and 'de-cluttered' with advanced functionality 'de-prioritized' and add-on support demoted to second-class afterthought - instead of the shining key feature advanced users value most. Basically, in recent years the Firefox UX and end-user features have been pushed by the substantial payroll needs of the Mozilla company to become more like Chrome and Safari instead of embracing its unique position as a tool for power users who value advanced features, customization and extension. And it was all for naught because Firefox has continued to lose market share while ignoring (and even actively alienating) its niche community of fanatically devoted power users.
