Since plugins cause so many problems in Firefox, should mozilla not have a different system of requirements in place before developers can offer plugins? They shouldn't consume so much memory - and there should be clear metrics that shows which plugin consumes how much memory, cpu and so on.
I think a new requirements to approve plugins would improve on the browser experience with Mozilla since plugins are sometimes quite necessary.
Also, it would be better if, like tabs, plugins should only be active when they are in use, not constantly running in the background and taking up massive memory/CPU.
Unfortunately, the add-ons review team is limited in what they can do. Already, it is only the Recommended Extensions that get detailed/ongoing testing.
Perhaps users can help report performance issues that arise in real world use? Of course, the add-on developer is in the best position to fix the problem.
If you haven't already found it, check out the about:performance page (type or paste that into the address bar and press Enter/Return to open it).