Some Firefox extension may cause crash of the browser or unwanted side effects when installed with other plugins or due to some bugs … etc. The MozillaZine have a list of Problematic extensions with the different extension concerned with their versions, the problem detected and the solution to fix it. Usually an upgrade to the latest version will fix the problem, but some extension are not suggested to keep them together such FlashGot and Filterset.G Updater. More issues are detailed in the Problematic extensions‘s page.
We’ll talk later with more details about this as we are preparing a big collection of Firefox extensions, we’ll try to keep it the more stable as possible so it doesn’t create any conflict with other extension or any unwanted side effects while browsing. Maybe it will be interesting in the future that some extension will be distributed by default in the Firefox distribution.