This is the only method of opening a new window in the same browser session or profile that actually works on my system. Attempting to open a browser session from the task bar "open new window dialog" or opening a new window while a current window/session is running will not open firefox in a new window or a new tab in the current session. The "Firefox is already running, but is not responding" error will occur every time.

Here is what I am running: Microsoft Windows 7, Service Pack 1, Fully updated to 4/10/2012 Fresh Install of Firefox 11. No add-ons enabled as a result of troubleshooting. I am a user with administrator rights UAS is disabled. To begin, this has been going on Since Firefox 10 or so. A few months now. I am able to open Firefox. My add-ons worked (ABP, Flashblock, ABP Element Hiding Helper, IETAB2, Downthemall, Greasemonkey, 4chan extension) had no problems. Firefox is set and has been set for the last two years as my default web browser. However Since approximately Firefox 10 or so, maybe before that, If I have a Firefox session open, and any other application attempts to open a session in a new window, I get the Firefox is already running, but not responding error." I attempted to follow troubleshooting advice already posted (look for parent.lock files left behind [none present], multiple sessionstore.js files [none present], create a new user profile, attempted to clear out the "read-only" permission in windows 7, however as soon as it is cleared, the read only permission returns.) As a last resort, I deleted the Firefox folder, the %APPDATA%/Mozilla directory in its entirety, then utilized CCleaner to remove any and all references to Firefox in the windows registry, then did a full re-install, no extensions installed right now. I checked preferences and have the option "open new windows in a new tab" checked off. The problem persists regardless if this option is checked on or off. This problem persists in or out of safe mode, even with the brand new install. Test Cases: All cases: Firefox is selected as the default browser for windows 7 Case 1: Open a firefox session. Utilize any other program that attempts to open a link to their website in a new browser window or new browser session. Experience "Firefox is already running" error Case 2: Open a firefox session. Right click on Firefox on the task bar. Select "open in new window". Experience "Firefox is already running" error Case 3: Open a firefox session. Select the Firefox Menu > New tab > New Window. A new window will actually open. This is the only method of opening a new window in the same browser session or profile that actually works on my system. Attempting to open a browser session from the task bar "open new window dialog" or opening a new window while a current window/session is running will not open firefox in a new window or a new tab in the current session. The "Firefox is already running, but is not responding" error will occur every time.