This is pretty damned impressive.

My question would be whether the work that's gone into this will also help the Windows/Mac versions of Firefox to load faster...

I've taken small example programs written in C# and pasted them into a java environment, changed the library references and I'm good to go. But doing that with 100k lines of code is quite another level. Still, I think the syntax and design similarities between C# and Java present a unique opportunity for this sort of thing... It would probably be prohibitively difficult to do with almost any other pair of languages.

The modern functional languages of the ML family may come closer.