Faisal comments on my last post that extensions regularly stop working with every new version of firefox. I had some of the same experience with Mozilla. I don’t know if they are alread doing it, but they really need to make extensions part of the test suites for these products.
The mozilla test suite needs to be organized to make it easy for extension authors to add acceptance tests for their extensions to the suite and the directory of extensions on mozilla.org should tell you which extensions are part of the test suite and guaranteed to work in subsequent releases and which are not. Mozilla.org needs to commit that it will make no release that fails the extension tests.