Some extensions will not work in Firefox 4 as they will not be compatible with the beta version of Firefox. There have been quite a few changes in Firefox 4 beta and many extensions will need to be updated to work with Firefox 4.

Having said that, there may be a problem with one of the files that stores details of installed extensions. Try deleting those files to see if it helps. First go to your profile folder using the method shown in the profiles article. Then close Firefox and delete extensions.cache, extensions.ini, extensions.rdf, compatibility.ini and extensions.sqlite. Now re-start Firefox and see if it lets you add or remove extensions.

Your above posted system details show outdated plugin(s) with known security and stability risks.

• Next Generation Java Plug-in 1.6.0_16 for Mozilla browsers

Update the Java plugin to the latest version.

• http://java.sun.com/javase/downloads/index.jsp (Java Platform: Download JRE)

Thank you both. I saw that the extensions i always use are not compatible with 4.0 so that isn't going to work for me. Since I can't get 3.6.8. to work any more I just uninstalled it and re-installed 3.6.5 and so far, so good. BTW I checked and I do have Java 1.6.0_16 installed already, so that wasn't the problem, but I'll make sure to always keep Java updated since it can cause some trouble. Thanks!

You're welcome

See also this MozillaZine thread for making some extensions work in Firefox 4.0

http://forums.mozillazine.org/viewtopic.php?f=23&t=1937759