There used to be MozREPL: https://github.com/bard/mozrepl https://github.com/bard/mozrepl but it never made it over the webextension/quantum changeover. I have

There used to be MozREPL: https://github.com/bard/mozrepl https://github.com/bard/mozrepl but it never made it over the webextension/quantum changeover. I haven't yet found a replacement, although I haven't been looking very hard recently.