I wanted to have Firefox 3.6 and Firefox 4.0 on my desktop for website testing purposes. I made a 2nd profile, and following a tutorial set my firefox shortcuts to: "C:\Program Files\Mozilla Firefox\firefox.exe" -p Firefox4 -no-remote "C:\Program Files\\Mozilla Firefox3.6\firefox.exe" -p Firefox3 -no-remote (is there a way to do this so that it automatically starts the correct profile, instead of me having to select from the profile manager?) Now if I double click on an HTML file, I get an error from windows saying: "Windows cannot find (filename). Make sure you typed the name correctly ..." And if I click on a link in outlook 2010, I get an error from firefox saying: "Firefox is already running, but is not responding ..." (firefox is running, but according to the task manager only once, and that one is responding normally as far as I can tell.) " and from outlook I get "General Failure. the URL was (url here). The system cannot find the file specified." I'm guessing this has to do with it not understanding which profile to pick? is there a way around that? (other than setting something other than Firefox as my default browser?) On the upside, I just installed everything on this system a day or two ago, so I have nothing really invested in either installation of firefox and have no problem with destroying profiles, as I have no data other than a day or two of browsing history and a few adons, neither of which are important to me.