Cannot Open local files. Terrible app, deleted it after losing invaluable time

Can't actually open XLS files with this extension. It also demanded I disable my AdBlock or it wouldn't let me use the extension, which is a;ready a huge strike for me (I've gladly disabled AdBlock for sites that kindly asked me to WITHOUT disabling their site for me, I resent it and just click close when forced), and after I tried that, it STILL gave me the "disable AdBlock" message and wouldn't work for me.

I'm more than happy to delete this trash off my computer, Mozilla should get rid of terrible plug-ins like this.