Txt file, file type problem.

I dl and installed and tried out both Mime edit extensions from the add-ons section so that I could add instruction re txt docts. Believe it or not, I need Firefox let an external application handle text files because of the nature of info I have in them that the text editor I use handles better. Besides which word wrap is dealt with in an editor whereas FF just uses long sentences. For these 2 reasons I made the change but though the TXT extension now shows up under FF's file types, Firefox continues to open any txt documents despite that.

How can this be fixed irrefutably? I really need to be able to do this.

Thanks much.