That's a drop marker to indicate where items that are being dragged will go. It is meant to disappear immediately after dropping, you could try doing another drag and drop operation to reset it. If you have any userChrome.css modifications, try disabling them by going to [https://support.mozilla.org/en-US/kb/about-config-editor-firefox about:config] and changing '''toolkit.legacyUserProfileCustomizations.stylesheets''' to '''false''' then restarting the browser.

Note that this would be about ''#PlacesToolbarDropIndicator'' in userChrome.css. *https://searchfox.org/mozilla-release/source/browser/base/content/navigator-toolbox.inc.xhtml#549