Hardware Canucks - How to force out of date extension installation for Firefox

Firefox crashes on launch.. twice. Panic. Step one: Re-install. SUCCESS! Hey, where's my theme?

If the many Firefox updates as of late have left you with a couple of your favourite extensions out in the rain, unable to be installed, cold and "incompatible" then this should help. As I guessed, and confirmed with a little research, most addons will work even when Firefox says they are not compatible due to a field in the addon's installer specifying a version limit, often a guess at when the addon could use an update but before it actually becomes incompatible.. But if the developer is deceased or moved on to greater things, you are S-O-L.

..but we can change the version limit!

I have found 2 methods for bypassing this: Neither worked, but one I was able to alter successfully.

You will need: 7-zip and your extension file.

Getting your extension file(usually .jar or .xpi)

Manually search for your addon on Firefox's database (they won't be shown by default since the cannot be installed)
Click the 'Add to Firefox' button.
Right click the 'Install Anyway' button.
Click 'Save link as..' and select your download location.

Changing the version limit

Right click the downloaded file within windows explorer and move over 7-zip's context field. Click 'Open Archive'.
The displayed files are what is compressed here. Install.RDF is the file we're concerned with. Open it with notepad.
Find the <em:maxVersion> field, and within those tags should be an out-of-date version number.
Change that number to something higher than the current version of Firefox.
Save changed and when closing the archive, you will be asked to update the archive. Yes.. Do that.

From here, open Firefox and use it to "File>Open file.." the addon file (the updated .jar) and it should install as usual!

Hey wait?? If there was no version limit field or similar in the install.rfd file, or the add-on failed to install, it may be genuinely incompatible.