hello, this sounds like a problem possibly caused by adware on your pc. #please go to the firefox ''menu ≡ > addons > extensions'' & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). #also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there. #finally, run a full scan of your system with different security tools like the [http://www.malwarebytes.org/products/malwarebytes_free free version of malwarebytes] & [http://www.bleepingcomputer.com/download/adwcleaner/ adwcleaner], which are specialised in adware and browser hijackers. [[Remove a toolbar that has taken over your Firefox search or home page]] [[Troubleshoot Firefox issues caused by malware]]

No toolbars installed nor any unusual add-ons. Have run several as-ware & malware sweeps to no avail. Also checked mouse drivers and tried different mouse types with no changes.

can you try to replicate this behaviour when you launch firefox in safe mode once? [[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems]]