Are there particular actions which bring on its appearance, such as opening a new window, etc.? There's a high likelihood is a lingering add-on, particularly, an extension. Perhaps it has a nonobvious name? Try this: Disable ALL nonessential or unrecognized extensions on the Add-ons page. Either: * Ctrl+Shift+a * orange Firefox button (or Tools menu) > Add-ons In the left column, click Extensions. Then, if in doubt, disable. Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step. Does that knock it down?