>easily deactivable
Each time I install Firefox, I end up wasting a non-trivial amount of time carefully working my way through the settings in order to disable the pile of defaults that are as 'easy' to deactivate as they are infuriating. When updating the browser, some of these changes get rolled back, and some of them become more difficult to adjust, either by being removed from the options menu, or by being obscured in about:config - maybe the name has changed for no reason, or maybe the behaviour now depends on an entirely different configuration value.
A particularly obscene case is the auto-updater, which has gotten so bad that I have literally given up on figuring out how to disable it. (At least on Windows; firefox-esr on debian is, so far, free of this insanity) (Ever noticed how forced automatic updates in software get pushed by security fanaticism, but mostly exist to steal your data and ruin your day while they're at it?)
You, apparently, see an easily-deactivable feature in a clear and extensive settings menu. I see an ever-growing eldritch labyrinth, built solely to prevent me from having a straightforward internet browsing experience, all the while the browser's performance is slowly but surely - and noticeably - worsening in the new versions.
Anyway, screw you, Mozilla. At this point I'm only using firefox because I don't see much of a choice either.