> people who advocated for them feel betrayed ...
Consider how strong an emotional response that is to a search engine change, etc., especially from people very familiar with IT and capable of adjusting to change (changing the default search engine, for example), and in a context where they see software changes frequently (think of hosted apps like Facebook!).
I don't think Firefox's changes are enough to explain that emotion; if you'll pardon the metaphor, they don't contain enough energy to explain the reaction; the energy must be coming from someplace else. I think the emotion is from someplace else and about something else.
Also, the comments raised above don't address the merits; they reflect only anger, which is consistent with my point. (In fairness, it may not be the context to review the merits of all those changes.)
I always thought of Firefox as a paragon of power-user design, and I loved it for that. In terms of hours of usage, I'm sure I've logged more with Firefox than with any other program in my life. It might not be an exaggeration to say it's the most important piece of software I have ever(!) use.
So this change, in combination with the overall direction Firefox seems to be headed, does evoke fairly strong feelings, and I really don't think they are "actually about something else".
Having said that, I do want to say that I am not angry, and I don't want to vilify anyone. I'm just concerned, and a bit sad, because the browsing experience I've grown up with seems like it won't be around much longer.