I have 21 different add-ons on AMO right now, and while it's kind of annoying to do so, it only takes me about 10 minutes to update the maxVersion on all of them every time a new version of Firefox comes out. Of course, the problem isn't that it's hard or takes a long time to do; the problem is that developers forget to do it.
Maybe it's time for Mozilla to move to using minVersion only, like Chrome does.
That requires a major commitment to backwards compatibility, which is a massive time-suck and reduces your ability to innovate. Look at Microsoft for an example of how nasty that can be.
It's not necessarily any worse than if the developer simply doesn't care enough to test it/update it. It might be better to use minVersion, then have plugins that use deprecated functions disabled.
Maybe it's time for Mozilla to move to using minVersion only, like Chrome does.