> 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.)