This spyware-behaving add-on installed itself and can't be removed or deactivated. The Windows app is installed too, by the admin, and i don't have admin access, which explains where the add-on comes from. But an add-on that installs silently without my consent, has access to my file system, collects website activity, downloads files and modifies download history and can't be removed -- that qualifies as malware in my perception.

Reeeealy useful thing