Technically it's harder to remain feature compatible if somebody ripped out the "modern BS" from the browser, because anything put into Firefox automatically becomes a de facto web standard. But they're by and large multimedia extensions which shouldn't be considered a necessary part of displaying hypertext.
I think if people want a programmable multimedia platform, that's fine, but they should build it separate from the world wide web so the rest of us can browse the New York Times without a 500MB web browser in resident memory, nor wait for the browser to stop the millions of operations it needs to reload the page due to all the ridiculous and unnecessary crap in the background.
Instead of forking the browser to strip out or redesign the unnecessary crap, i'm suggesting we take pruning shears to Firefox itself, lest we get yet another new tool that tries to replace yet another bloated, antiquated piece of crap.