What version of Firefox are you running? With the release of 0.9, the extension manager changed significantly. You might be installing a 'new' extension onto an 'old' build.
And, 0.9 was kind of borked, 0.9.1 is better. There's going to be a 0.9.2 any day now, for a security fix.
I'm using
nightly/2004-07-07.