Firefox just came out with their update from v4.01 to v5. This will, for the time being, make a lot of themes and add-ons incompatible.
To get around this, install Mr Tech Toolkit. But, the latest version of this does not work with FF5. To make it work (and make your other add-ons work), download (don't install), v184.108.40.20600 here.
Next, open this file with 7zip (get that here.)
Once you have 7zip installed, right-click on the toolkit.xpi file, choose 7zip and Open Archive.
Extract the install.rdf file.
Open it with Notepad.
Look for 2 locations of
and replace these with
This will force this add-on to be compatible with all FF versions up to just before 8.
Save this file and close it. Now, drag it into the 7zip window, archiving it and overwriting the original file in the archive.
To make sure you did things correctly, double-click on the install.rdf file and check to make sure the maxVersion>4.* has been changed to maxVersion>7.*
Once you've successfully changed this, open Firefox.
Open Add-ons (orange button at top left, then Add Ons, or, compact Menu button, tools, add-ons, or Tools, Add-ons)
At the top right beside the search field, there is a button with a down arrow on the side of it. Click on this and choose Install Add on from file.
Then, navigate to the file toolkit.xpi.
Close Firefox again
Open it again.
The problems should be solved.
(You might need to close and open Firefox one more time.)
Note, if the Add-ons manager page can't be closed, press Alt+F4 to close it.