This is obviously due to the poor extension API that exists in FireFox. I wouldn’t blame Google for writing the bad toolbar code base because in my opinion they have the best software engineers on the planet.
Working with Google will create the same issue that Microsoft experienced with IE back with IE 4.0. Once M$ opened up IE to being extended by several plug-ins this is when the security holes began to multiply. The FF Developers will have to make the extension API much more solid then it's current state and by doing so they'll open up a huge can of worms. Extensions will severely hurt FireFox and its hopes of it having the leading browser Market share.
The FF Development team should stay away from extending the browsers core functionality of BROWSING and should just make a solid browser or they'll be even more buggy then M$.