[mook]

The first Firefox release with multiprocessing work was one of the 3.6 ones where they shipped out of process plugins in a patch release. Judging by Wikipedia, that was 3.6.4 in 2010 (look for OOPP). I thought it was later…

Having to break legacy extensions due to internal changes is understandable. It would have reduced stress for the users if the new API was shipped first, then the changes made and the old one broken, though. Heck, they even thought of it first and did that - you may remember Jetpack. It never picked up steam because the APIs were too limited and too many people ended up resorting to the old APIs, though. Even now with WebExtensions they're shipping lots of new API extensions want in the same release that drops support for the old ones…

Oh, and they had multiple breakages as more and more things went out of process, not one giant one. That's life, really; things take time to build. I believe part of the pushback is that people just finished porting to e10s only to find that their new code will need to be rewritten again for WebExtensions.