Uninstall the extension.
Download the latest version of the extension.
It’s really a ZIP file so rename it from Extension.xpi to Extension.xpi.zip
Unzip Install.rdf into the same folder as the .zip file.
Find the line containing the word maxVersion. Change the number between the angle brackets to a number equal to or larger than your current Firefox version.
Save the file, then add it back into your Extension.xpi.zip file, overwriting the original Install.rdf.
Rename it from Extension.xpi.zip back to Extension.xpi.
Drag and drop the file into an open Firefox window.