I am not familiar with Web Extensions, but I am familiar with the fairly common event in which application vendors severely change functionality, with significant downstream effects. Such behavior seems more common amongst open-source products. Consider Python 3. Asterisk's scripting language. Perl 6.
Incompatible version changes drive away users and developers. The uncertainty that comes along with a planned update in which compatibility is not guaranteed makes us all shy away from developing in that application at all, not just on the old version.
Microsoft always seemed to do a good job at maintaining compatibility over its versions. Most applications that used to run on Windows still do, although there are a lot of exceptions. Python's change to the print statement broke everything, though the fix was easy; most of the time the fix is not easy - and even when it is, nothing works until someone gets around to the fix.
I appreciate the need to move forward, and sometimes you have to break stuff to make it better, but these incompatible changes just destroy momentum. It's a shame to see it happen.