[Solved] Installing an add-on on an old version of firefox updates the version

I'm trying to debug a web-page that doesn't seem to work in FF4. I had FF5 installed on my machine. I downloaded and installed FF4.0.1, in a different directory than FF5, created a new "FF4" profile using the profile manager, and from the FF4 install directory ran "firefox.exe -P FF4 -no-remote". The browser window that came up had the "welcome to firefox" stuff, and the "About" menu said 4.0.1, so I thought I was fine. Then I went to install the DOM inspector. It ran OK, but after the reset it said "Your Firefox has been updated", and I was running FF5. Even when I go into the FF4 installation directory, and run "firefox.exe -P FF4 -no-remote", I still get FF5. How do I install add-ons in FF4 without having the process move my FF forward to FF5?