@cor-el, Thanks. Very helpful info on how to view the file. Interestingly though, the file only lists the two recognizable entries for the mailto handler in about:config gecko.handlerService.schemes.mailto.*. The file does not list any additional entries which I had sort of expected to see corresponding to the two additional, unidentified entries in about:config gecko.handlerService.schemes.mailto.*. My only thought is to delete those two extra (?) entries in about:config gecko.handlerService.schemes.mailto.*, but I'm hesitant to do that without being certain of exactly what I would be deleting. As discussed with RichardInEngland, the issue is not causing problems; it is just kind of annoying. I certainly don't want to break something else for a mere annoyance. Still, it sounds like a possible bug, so I hope it will get fixed at some point.