There is a firefox addon that is not supported by the latest version installed on my computer.
I know there is a way to use this by editing the xpi file,but I can't even download the xpi files from the mozilla addon site (the "get addon" button is disabled)
Next (and more specific to your question) you need to avoid the site protection: use the "older version" in the bottom of the page (credit: CGA) or create a mozilla acount, you will have a "ignore version check" link in the button (credit me)
Hit up the All Versions link at the bottom. Check the box to install the experimental add-on. The Add to Firefox button should go from gray to green. Now right-click on the button and choose Save Link As. Now you can hack it to work with your version if necessary and then drag the updated XPI into your Add-ons box under the Extensions section.
There is now a better way to do this: Use the Add-on Compatibility Reporter, an extension developed by Mozilla themselves. In addition to allowing any extension to run, it lets you give feedback about whether it works or not.
Help Mozilla make sure your favorite
add-ons get updated for upcoming
Firefox releases by using this
extension to report whether they still
work or are having some issues with
alpha and beta releases.
Provides the tools needed to install
and manage extensions and themes
locally and tons of other features.
This extension provides
multi-extension installation support,
hacking capabilities to the
Extension/Theme manager windows,
features to find and troubleshoot
Extensions/Themes Build, GUID and