You'd have a point here if Firefox hadn't started blithely breaking everyone's add-ons by bumping the version number about 5 times per week. Okay, that's hyperbole -- I know it wasn't that often, but the point is that a "rapid release schedule" comes with real costs.
If you have 500MM users, and the mean user time to adapt to a trivial new release is 5 minutes (a gross underestimate if add-ons have broken), you've just wasted 16 million hours of user time. Just because that cost doesn't appear as a line item in the Mozilla Foundation's budget doesn't mean it isn't real.
I used Firefox for years because of the rich add-on community. When it started being more trouble than it was worth to update the add-ons (or find/write new ones, if the old one hadn't been updated), I switched to Chrome. It's unlikely that I'll be back.