A bad reputation sticks

As you said, the big ones that everyone uses have the resources to check and update. Many of these update even more frequently than firefox because of their huge user communities.

But a lot of the small specialty extensions are gone. I wouldn't be surprised if half the extensions that existed back in Firefox 3 days are gone at this point because of lack of maintenance.

Mozilla forgot their strengths, most people I know aren't out reading browser benchmarks, they are using the software for features and UI and that's what they keep breaking/changing.