Test the extentions one at a time (Score 2, Informative)

Every extention has an install.rdf file which contains the version numbers it works with, if you go to your profie dir and then extentions you can move the extention folders out, edit the rdf files in notepad then restart firefox - it will have no extentions, close it, then move all the extention folders back one at a time restarting firefox everytime, that way you are only adding back extentions that you know work, but just haven't been updated by their developers to install. This can also be done by renaming the .xpi files to .zip then opening editing then returning to .xpi and installing. If you are comfortable doing this i view this as safer than just allowing all addons as if something was crashing firefox you would have no idea, where as if it did you would know and you would live without one extention rather than one.