The problem of  that extension is in fact are already been disabled, because no longer compatible, but remains in the extension list until to an update, where can be automatically reinstalled. This situation seems irreversible because the “uninstall” button is no longer activated. If you wish you can delete them with a little surgery on the registry of your Windows. So follow the instructions at your own risk.

Open Regedit (start -> run -> regedit [return])
navigate in your left until you find the entry:
HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Firefox\extensions
First we prepared a good backup of this key, right mouse button, choose “Export” to save where you like (to restore just a simple double click on the saved file).
Now focus our attention on the right side of Regedit, you should see the entries “locked” in our case it is the voice bkmrksync@nokia.com and just above a path that clearly draws on .NET framework from Microsoft.

We need to eliminate it, with the right button on the item you selected, and then “delete” command.
Start Firefox and “magically” extensions blocked are missing from the list.