Can you attach a screenshot? *https://en.wikipedia.org/wiki/Screenshot *https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem *use a compressed image type like PNG or JPG to save the screenshot *make sure you do not exceed the maximum size of 1 MB Is this tab opening in the sidebar like you can open the bookmarks (Ctrl+B) or history (Ctrl+H) in the sidebar? ---- You can check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. You can use this button to go to the current Firefox profile folder: *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox ----- Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem. *Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance *Do NOT click the "Refresh Firefox" button on the Safe Mode start window *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes See also: *https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Thank you for your suggestions, however, I found the answer myself, by accident. Someone who uses this computer, and will remain nameless, pinned the tab, that's why it wouldn't go away. I unpinned it by right-clicking the tab to open the drop-down list.