Installing more than one extension
From MozillaZine Knowledge Base
This bug is fixed in Firefox 0.9 and above. Users of versions before 0.9 are advised to upgrade to the latest version. If you choose to stay with Firefox < 0.9, here is the solution to the bug:
You can occasionally encounter problems with Firefox when installing more than one extension at once. When you install the second extension you may be asked questions relating to the installation of the first extension.
While this problem is intermittent, its effects are not yet well known. To avoid this problem restart the browser after installing each extension.
Problem: extensions will hang in the download and an alert saying "Software installed" will appear, although the extension has NOT been installed (Firefox 0.8).
Solution: Creating a new profile and installing extensions using that can work around that problem. The extensions will work in the usual profile too. However, you'll have to be careful about installing extensions in the applications directory rather than the profile directory.