Made it impossible to use add-ons like Tab Groups, making impossible to use the browser with many tabs open (just like Chrome, but you can't open many tabs in it anyway because it eats so much memory per tab).
IIRC, the author of Tab Groups threw their toys out of the pram when told the extension model was changing and that if they needed a feature, they should post on bugzilla. I don't know quite what's happened since then.
EDIT: Looking at bugzilla, looks like the author of Tab Groups decided he was going to put together a report about a year ago, and then never did, so Mozilla doesn't even have any documentation of what functionality would be necessary to implement it, and there's currently nobody interested in implementing it.
It's not "throwing toys out of the pram" when add-on developers are forced to rewrite their entire code every damn year, and this time using an incredibly limited API from goddamn Google Chrome of all places. There are a plenty of bugs open in Bugzilla to implement necessary APIs so the old add-ons can work. The problem? Mozilla devs are closing them as WONTFIX because they're lazy and don't want to implement any more functionality than Chrome feature parity.
Also, Mozilla knows perfectly well which functionality is needed to implement Tab Groups because it was an actual feature of a browser before they decided "it should be a plugin". And now it's impossible to even write a plugin for it.
It's not "throwing toys out of the pram" when add-on developers are forced to rewrite their entire code every damn year, and this time using an incredibly limited API from goddamn Google Chrome of all places.
It's one or the other.
Either the API gives very deep control, and it must evolve as the browser evolves, or it does not, and it can stay stable.
Experience says that Firefox was on the wrong side here by offering complete access, thereby either blocking the browser to evolve, or forcing add-on authors on a compatibility treadmill, effectively getting the worst of both worlds.
Maybe it's possible to do better. But we haven't managed to do so, and neither has our competition. So it's probably not easy.
To circle back to the original question (what's it like to work at mozilla), it is a pretty great place as long as you don't mind being the subject of uninformed and hostile comments such as the one above.
EDIT: Looking at bugzilla, looks like the author of Tab Groups decided he was going to put together a report about a year ago, and then never did, so Mozilla doesn't even have any documentation of what functionality would be necessary to implement it, and there's currently nobody interested in implementing it.