If you decided to deliver Firefox for very old browsers, you could emulate canvas (as example) by matrix of 1x1 pixels of divs. Maybe it wouldn’t be very smooth, but should works.
I also read about GTK4. GTK4 will use css to style elements more often and broadway backend wouldn’t render data on server, but instead send styled html elements. I know Firefox isn’t ported to GTK3 yet, but if you decided to use GTK4, my idea will be a lot of simpler. Just compile GTK4 with broadway backend to ASMJS, modify it and port Firefox to GTK4.
If I’m wrong, just tell me. I don’t know each technical details about ASMJS, HTML5, ECMASCRIPT and GTK/GDK/GLIB. Maybe it should introduce threads, so ASMJS wouldn’t be a good target, but better could be Java for example. Posix threads aren’t supported yet by all of browsers, so older wouldn’t support it.