I have created two profiles. One to run firefox 4 and one for firefox 3. I run both for testing puposes in a webdevelopment environment. The profiles are called firefox4 and firefox3 respectivally and these are the only profiles. I've changed the shortcuts to read path/to/firefox4/firefox.exe -P firefox4 -no-remote and path/to/firefox3/firefox.exe -P firefox3 -no-remote I can now start both versions from there shortcut at the same time. But when firefox 4 is open (I want to use this as the default browser) and i click on a link in another program (i.e. a link in an email openened in thunderbird or from a twitter-app) I get an error: Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system. I thought that with te -no-remote options problems like this should occur. Is there something I'm missing or is there a fix for this problem?