User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.3) Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3/2008092417 Firefox/3.0.3 Unable to launch the browser for the second time. Reproducible: Always Steps to Reproduce: 1.Launch/open the Mozilla browser. 2.Enter any valid URL and Enter. 3.Close the browser. 4.Launch/open the Mozilla browser again. Actual Results: Mozilla browser should launch/open again. Expected Results: Close Firefox pop-up with following information "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" is displayed.

Does it also happen if you wait for a while? It could be (with Firefox 3.x on slower systems) that Firefox is still in the procedure of closing.