For firefox 3.6. Rated 5 out of 5 stars
For firefox 3.6. Here's how:
1. Download the .xpi file to your computer.
2. The .xpi is a file like a .zip, Open it using a zip application like 7zip.com.
3. Edit the install.rdf file and change the max version to 3.6.*
4. Save, and then open with firefox to install.