It could be a feature of one of your extensions. You can review (and, if appropriate, disable/remove) extensions here: orange Firefox button (or Tools menu) > Add-ons > Extensions category If nothing seems related, you could post a list of your extensions. The easiest way is to open this page -- Help > Troubleshooting Information -- and at the upper left click the ''Copy text to clipboard'' button. Then paste that into a new reply, and delete everything after the extensions list. If you do track it down, please report back on what it is so other users who find this thread in a search will benefit from your research.