one can "disable" it by making %APPDATA%\Mozilla\Firefox\Profiles\*.default-release\extensions\com.abr.startapp@adminbyrequest.com.xpi unreadable by the user (permissions, replacing the file with a broken symlink, or other methods).

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