In case you are experiencing a problem installing Firefox extensions on your upgraded Firefox 1.5 version, here is the solution:
Firefox version 1.0 had an option of disabling software installation (themes / extensions)withing the menu options.
If you disabled this feature, Firefox remembered your selection when upgrading to version 1.5 .
The problem is that controlling this feature is no longer possible through the options menu.
You will need to type “about:config” in the address bar in order to access more advances features.
Look at the Preference column for “xpinstall.enabled” and double click it, making sure the value (in the value column) changes from false into true.
That’s it, now you are able to install new themes and extensions.