User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.15) Gecko/20110303 Firefox/3.6.15 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.15) Gecko/20110303 Firefox/3.6.15 When a popup window (in particular perhaps a JavaScript dialog box) is opened in a tab in the same window (by typing its URL in a new tab in the original window once it's opened in a new window), that tab tends to persist forever as long as the browser is running. If there was just one page initially opened in the browser, for example, and a popup is now opened in a new tab (thus resulting in two tabs), these two tabs will stay open forever. If you try to close any one of them, a new tab immediately opens (as if there were no other tabs present). Reproducible: Always Steps to Reproduce: 1. Start the browser and open a website containing a JavaScript popup link. 2. Click the link to open the popup, copy the popup window's URL, close the popup. 3. Open a new tab in the original window, paste the copied URL, and press Enter to visit that link. 4. (Clearly, there are now two tabs opened in the browser.) Close one of the tabs by pressing the Close (x) button. Actual Results: The intended tab closes and a new tab immediately opens up. Expected Results: The intended tab should close. I'm using the Chromin Frame and the Tab-Mix Plus add-ons along with the others that do not seem to have to do with the tabs. Default theme.

Could you see if the issue occurs if using Firefox in safe mode: http://support.mozilla.com/kb/Safe+Mode How about with a new, empty profile? http://support.mozilla.com/kb/Basic+Troubleshooting#w_make-a-new-profile If it still occurs, please try using the latest nightly: http://nightly.mozilla.org/ Also, please give an example URL where this occurs.