Disclaimer: I recommend not following this tutorial unless you create a new profile for Firefox (or your other Mozilla based browser) first. Not doing so could result in loosing your bookmarks permanently for example!
The following introduction can also be read at...
Firefox Profiles Introduction
If you do not know how to access the profiles window then Windows users should go to the Firefox shortcut's properties and change the target from...
|"C:\Program Files\Mozilla Firefox 2.0\firefox.exe" |
|"C:\Program Files\Mozilla Firefox 2.0\firefox.exe" -ProfileManager |
Opening this link if Firefox is still running will merely open a new window. When you open this shortcut and either nothing happens or a new window opens but you get no small window offering a choice of profiles you'll need to go to the Windows task manager (CTRL+ALT+Delete) and manually end the Firefox process.
Creating a new Firefox Profile
If you're going to skip this part you are putting your bookmarks and other personal settings at risk.
Click on the Create Profile... button and be sure to name your new profile something such as Firefox 3 Test.
You can install an addon/extension in Firefox 2 using this profile and then loading Firefox 3 or by following directions later on without using Firefox 2. I highly recommend labeling your profile name to correspond with the version of Firefox you plan on using. I have profiles going back to Mozilla Suite 1.0 (though only Firefox 0.9 will show up on this list while Firefox 0.8 will only display when you manually load Firefox 0.8). I do not recommend loading one version of Firefox and a profile created for a different version of Firefox.
Backing up Firefox Profiles
Once you've created a Firefox profile specific to the version you intend to use it with I highly recommend backing up those profiles. They will typically be found at...
|C:\Documents and Settings\*YOUR_USER_NAME*\Application Data\Mozilla\Firefox\Profiles |
Download and amend Extensions
Visit the website(s) for your favorite extension(s) and right click and save the XPI file some place on your computer.
Secondly rename the file (press F2 when selected or right click on the file and then choose rename) and rename the file to have a ZIP extension instead. If you do not see the file extension or it disapears when you change it from XPI to ZIP open any folder in windows, click on the Tools menu, click on Folder Options, click on the View tab, and then uncheck Hide extensions for known file types.
Next you'll need to extract the file install.rdf. Search for the line...
You can change this to any version you want and newer versions such as Firefox 3 or Firefox 4 will allow the extension to install and will not disable it upon restarting after installation. However this does not mean the extension will work, much less crash Firefox. Some, most, all, or none of your extensions may work.
Be sure to update the file back in to the ZIP file. Rename the ZIP extension to XPI. Lastly manually open the XPI file with Firefox (or you can manually associate it within Windows itself). From there on it's just like installing any other extension.
I hope this helps. Remember to not use your default profile.