With release of Firefox with new look and other security features (CSP,HSTS) I couldn't resist myself from updating my current version 3.6 to 4.0. But, to my disappointment, lots of plugins which are necessary to me for carrying out my day to day assessment were not supported in FF4.0. I can't wait for them to extend their support to FF4.0. I got a nice trick to enable them.
Go to the following path on your Windows XP machine: C:\YourUserDir\Application Data\Mozilla\Firefox\Profiles\8l13fo9x.default\extensions There you will find folders named as your extensions e.g for Firesheep, its "email@example.com". Go inside them, you will find one install.rdf file. Open the install.rdf file and edit the em:maxVersion tag, change the value from 3.6 to 4.0.*. Restart the browser and your plugin will be enabled ! Reference:http://research.zscaler.com/2011/03/make-your-old-add-ons-work-with-firefox.html