Rated 5 out of 5 stars
It's easy to update any Add-on to work with any version of Firefox.
* Download and save the Add-on to your desktop (or anywhere else where you can find it later)
* Locate the saved Add-on using Windows Explorer (or similar application)
* Rename the extension from .xpi to .zip
* Extract the file "install.rdf" to your desktop (or anywhere else where you can find it later)
* Open "install.rdf" with Notepad (or a text editor of your choice)
* Look for, or search for, "<em:maxVersion>". Here you will see something like "<em:maxVersion>xx.*</em:maxVersion>" (where xx is some version number)
* Change xx to the current version of Firefox you have installed. Be sure to have the .* after the version number. This will make the version available to all updates for that version. You can also increase the version number beyond your current Firefox version if you want to
* Save the changed "install.rdf" file and put it back into the .zip file overwriting the existing file
* Change the extension of the Add-on from .zip to .xpi
* From within Firefox go to your list of Add-ons and click the "Tools for all Add-ons" button (might look like a gear)
* Click "Install Add-on From File..."
* Navigate to your desktop (or wherever you saved the Add-on file you just worked on) and select it for installation
* After the Add-on is installed restart Firefox and the Add-on should now be working
I just updated this Add-on to version 20.0.1 and it works just fine.