Mozilla asks for user data and people who do not opt-into that are not contributing data. Me being one of the many who do not-- I suspect intermediate and advanced users comprise the majority of this group. This means their data of people not using things like menu bars because they getting metrics from the most daft users of firefox.
Good designers will use metrics only as a factor not as a mindless system to think for you. Simplistic metrics are a whole issue in themselves along with improper use of statistics (on metrics) which is a common problem as well. Menu bars are never used heavily but they are extremely useful - of all times, in 2014 when phones have more screen space than a desktop did in the 90s we suddenly become obsessed with screen space??
Great designers also will accommodate advanced users and the large base of existing users by not arbitrarily pissing them off. Necessary changes can be done more gradually along with instructions on how to change the feature. (like making sure the user knows how to get to menus when you killed them... and to not foobar the pop-up menu version of the menubar... proper grouping and hierarchy make large things easier.) Also the current situation of "don't make me think" is likely a fad in the design world; I hope that users want to use their brains effectively in the future; otherwise, Edward Tufte etc. are irrelevant as we devolve.
If Mozilla wants to REALLY be a community they will let users choose and try something democratic, such as opt-in or opt-out of a major interface change. Since opt-in would never gain a majority of the users on these recent changes; the designers would naturally push for an opt-out policy but at least they could measure their failure by making opt-out easy to do (like force the user to use it for a few months before presenting the option.) At least then users at all skill levels feel empowered and PART OF SOMETHING (mozilla could even use the opportunity to leverage altruism and promote an organization image unlike the top-down corporate browsers.)
FURTHERMORE, it doesn't matter how many more daft users you have over the advanced users. Your software is not default like IE was. Users install Firefox because of people like slashdot readers. I have brought mozilla 100s of users and I can take them away, some already left for Chrome anyhow... but many do what their nerd or IT staff tells them to do (or whomever sets the default.)