My guess is that power users are picky and opinionated. If one of their favorite features gets cut back or dropped they may loudly complain on the way out.
Mainstream users don't care about any of it. The only reason Firefox or even Netscape were ever mainstream was because they were undeniably better than alternatives. User advocates got the word out, but it was the clear advantages carried the day.
Now that browsers are all mostly the same for the laymen there's no reason to change the default. Or if they change it's only to whatever's most familiar.
Mainstream users don't care about any of it. The only reason Firefox or even Netscape were ever mainstream was because they were undeniably better than alternatives. User advocates got the word out, but it was the clear advantages carried the day.
Now that browsers are all mostly the same for the laymen there's no reason to change the default. Or if they change it's only to whatever's most familiar.