How to change URL handler for firefox?
To fix this, you need to delete/change the association within Firefox.


Go into Firefox Preferences (hamburger menu in upper right hand corner of window or press ⌘ Command,)
Scroll down to Applications.  You can either scroll through or search for the association you wish to modify


Delete the association; the next time you click on the link it will ask you what you want to do.  Or... change the association to the application you wish to be opened.