This is old news to a lot of people, but it's a useful tidbit that I'm sure a lot of people could use.

If you're trying to install a Firefox extension and Firefox tells you that it can't install the extension because your version of Firefox doesn't support it, there's a possibility it'll still work.  You just need to trick Firefox into thinking it's compatible.  Note:  You try this at your own risk.  If you hose Firefox, your computer, and/or your relationship with your significant other, I don't want to hear about it.

Save the extension's .xpi file to your local machine (right-click and "save link as...").  Rename the file with a .zip extension.  Open the zip file and extract install.rdf.  Open it in your text/xml editor of choice.  Look for the tags em:minVersion and em:maxVersion and edit as appropriate for your version of Firefox.  Save install.rdf and add it back to the zip file, replacing the original.  Rename the zip file to its original filename with the .xpi extension.  In Firefox, click "File|Open File..." and open the .xpi file.  It should install now.

If the extension installed but doesn't work, it probably genuinely isn't compatible.  Uninstall the extension.

I've had cases where Firefox complained about being unable to install the extension because of an invalid signature.  If this occurs, delete the META-INF folder from the zip file.  That folder is what contains the signature information.

Good luck!