Bumping XPIs ~ Time-Honoured Technique Rated 5 out of 5 stars
Well, in response to Gaz, actually relation was a bit closer. There are really 2 ways; one is to get the tiny free app that does the job for you. Google is your friend.
However, to bump any XPI, you right-click to download it (to your desktop is a good place for this operation). Then right-click your desktop download and open it with 7-Zip's file browser. (You don't have 7-Zip? How long you been born, anyway? Well, go to SourceForge and get 7-Zip, give it total default in all its tweaks, and get back here.)
OK, so in 7-Zip, right-click the Instal.rdf and choose EDIT. Use Notetab Lite, never never Notepad. (You don't have NoteTab Light? Aw, C'MON! You born just yesterday?? Get it. Gee whiz.)
OK, just change the em:max to whatever. If it's an active XPI, just bump it a little bit, so you don't miss out on updates coming in awhile. If it's old and pretty well dead, bump it high, so you don't have to do it again for awhile. I put Download Sort up to FF 3.2, myself. Adding an asterisk [*] at the end gives extra latitude.
Now, with your Add-Ons window visible on your desktop, but Firefox itself diminished [-], drag the altered .XPI anywhere into the Add-Ons window and wait for the magic.
(Note that you can store all your XPIs in your files and drag the whole bunch of them onto an Add-Ons window this way. A lot of guys therefore maintain their own XPIs Archives.)