Several times on different computers (both Win10), while editing a newsletter, the action of selecting text then clicking italic and bold has resulted in FireFox shutting down abruptly. Italic only doesn't do it & bold only doesn't do it. FYI.
Probably it's a problem of your firefox, maybe due to an installed extension. There is nothing we can write in a webpage to crash a browser. You should report that kind of crash to the firefox developers, they have an utility to report the crashes.