The article phrases it in a way as if to indicate that Firefox has to choose between being slim and having good (meaning bug-free) plug-ins. Why not have official Firefox-supported add-ons, distinguished from those made by users, which add functionality such as spell-checking and page-caching? These add-ons could even be included in the default installation, with the ability to remove them if desired.
If they did this, the core of Firefox would be slim, with additional functionality provided by plug-ins, just like the original vision, and all the great functionality that has been added lately would not be sacrificed.
I found it handy to do a binary chop on my installed extensions. Turned out that only 2 caused any significant memory leaks, so I disabled them until they updated them and fixed the leaks.
If they did this, the core of Firefox would be slim, with additional functionality provided by plug-ins, just like the original vision, and all the great functionality that has been added lately would not be sacrificed.