It's not going to inconvenience you that much, and the proof for that is that this has always been the case: extensions would never run on e.g. addons.mozilla.org. This makes sense; you don't want extensions to trick you into installing other extensions, for example, or to hijack your sync password.

It looks like the main change is that this actually loosens this restriction: it looks like some trusted extensions from now on will be allowed.