this could be a huge deal a lot of web users
Oh, I agree with that. I think that's kinda the point of the title.
But you can bet that news of this problem will be quickly to spread when Web app developers start notifying their users to switch to a browser that "just works". And I don't mean just Safari. There are many good browsers around and web app developers will not delay their users from using those apps for long. That costs developers money, which is often said to "speak loudly"!
There are already standards and if you want folks to use your browser (which is
not where the money is), you'd better make sure it uses those standards. Immediately, transparently and easily.
I suspect Mozilla will find and implement a solution quickly. If the web can't handle version number over 99, then just start over at 1 or 10 or (wait for it...) the two digit decade!