How then can you upgrade to the latest and greatest version of Firefox without having to sacrifice missing add-ons? Can you force your add-ons to install (at least) and check functionality with Firefox? The answer is yes, that's exactly what you need to do and here's how.
First, download the add-on to your hard drive. Right-click on the "install" button and choose "Save Link As.." (or another name for it). You will be downloading a file with .xpi as extension. And, chances are, it will not be recognized by Windows.
Next, open the .xpi file with your favorite archive program. WinRAR and 7Zip are tested to work, so use either one. Extract the file install.rdf inside that .xpi archive. It is advisable to save the .xpi file on the Desktop for ease of use later.
For our procedure we will play around with FasterFox add-on which is compatible only up until Firefox 2.0.XX versions. So this will never install with Firefox 3.XX.
(NOTE: This add-on was chosen for the purposes of illustration only. But if you want to use it, by all means do so.)
Open install.rdf in your favorite text editor. Look for the string "maxVersion" (see below) and change the value to 3.5. Save the file and re-insert it to the .xpi archive.
Launch Firefox and drag the .xpi file from your Desktop to the Firefox window. It will then install and prompt you to restart Firefox. Once restarted, viola! your add-on is now installed. This, of course, does not guarantee that your favorite add-ons will work exactly as expected. It just allows them to install. Others may work and others may not. Therefore, use this guide at your own risk.
But if your add-on is compatible with Firefox 3, chances are it will be compatible with Firefox 3.5. Again, no guarantees.
