Basically, hover over a link on a page in Firefox and wait for the tooltip to display, then Alt-Tab out of Firefox to another program, and more often than not, the tooltip will remain on the screen. To remove it, you must go back to Firefox, and move the mouse.

As of 2023-11-29 this is still happening for me on Fedora Linux 38, running Gnome 44.5 and Firefox 118.0.2.

As a temporary, if not sometimes inconvenient fix, I’ve set browser.chrome.toolbar_tips to true in about:config as per here