When I worked on Firefox, most of the changes happened exactly because user research determined that users wanted them and/or that not having them hurt the product. We changed the tabs at least once because users thought that the old shape of tabs made the browser feel slow (true story, sadly). We changed the add-on API (after having warned add-on developers for at least 6 years) because the old API was incompatible with multi-threading, multi-process, sandboxing, which in turn was really bad for both performance and security.

I'll absolutely grant you that Mozilla hasn't been very good at communicating these choices, but again, the sheer hostility of tech crowds is exhausting.