Yep. If you make a change that upsets nearly 40% of your userbase, can you really act surprised when you hear people complain about it? Even if you go with the comparatively tiny 5% estimate of users who are 'unable' to switch, that's still a number that's in the millions.
This is the sort of thing that Google does all the time - optimize for the 90% at the expense of the remaining 10%. I think a lot of the more extreme backlash is because of that vocal (but not insignificant!) minority which is disappointed by the perception that Mozilla is going down the same route as Google (and all the other tech behemoths before them).
What? You can't describe 38% of your user base with the word "only". This should read "a whole 38% of Firefox users ..."
I use Firefox because I trust it better than chrome, but I need those extensions I am using.