One thing that Mozilla doesn't get (and engineers in general, I ween) is that changing things imposes a cognitive load on the users.
I'm used to Firefox, it does what I want and doesn't require my attention very much. The major reason I don't switch to Chrome or any of the other browsers is cognitive load: I'd have to learn an entire new way of doing things. Different looks, different icons, different behaviour... it would take hours to figure out the new system, many minor "how can I get it to do this..." moments amortized over the next year.
Every time Firefox changes, it's a distraction. Something to notice, figure out, and get around. For me this time it's the offline cache system - no amount of fiddling with the options or about:config will cause the system to save tabs on program exit and load those tabs anew on start - the weather *has* to show yesterday's page on program startup(*).
The previous issue (for me) was putting the window rendering in an external thread, the upshot was that cascading menus took several seconds to render. Click, count to three, then see the bookmarks... move the cursor, count to three, see the selection bar move down. Setting the about:config option to undo this caused Firefox to crash on every boot, but un-setting "use hardware acceleration" fixed that. (My dad is *totally* going to figure that out and not move to Chrome instead.)
All this "OMGWTF we need to be like Chrome!!!" and "OMGWTF we need a chicklet interface" is driving users away from the system. For every change, a number of users say "screw it, I'm moving to $OtherBrowser".
Changing behaviour at all is stupid, doing it once a month is ridiculously stupid. They're thinking in terms of "how can we add more functionality" instead of "how can we attract and keep users".
Pro tip: adding complexity to every little feature does not necessarily make your software more popular.
(*) To be fair, I've only tried 6 of the 64 possible combinations of options that might affect this (in Options->Privacy and about:config). It might be a simple fix, I just need to uncover the right combination of options to do it.