Firefox goes insane when a web page tries to send Email.
Firefox is my default browser. Many web pages try to send Email through the default browser; one example is WeatherBug when you click on feedback. When I do that (on Chrome, incidentally), an infinite loop seems to be created with mailto because I don't have Firefox set up to send Email (and don't particularly want to do so). When this occurs, I have to quit or sometimes force quit and reopen Firefox. It would not bother me if a single tab were created with an error message, but many, many tabs are created in that infinite loop. I suspect that this is a bug in the way Firefox processes such mailto requests.
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I should add that although I am running Beta Catalina (release 10), I have observed this issue for a few years when I was running production Mojave as well. That's why I think this is a Firefox bug and not a Mac bug. It is absolutely reproducible. Each time a web page tries to send Email via Firefox, Firefox goes into this loop and hangs.