You mean what was done in IE8 six months before Chrome? https://blogs.msdn.microsoft.com/ie/2008/03/11/ie8-and-loose...

More seriously—it's somewhat sad to see Chrome get all the credit for this, when it's undoubtedly the case that both IE and Chrome were working on this at the same time.

Chrome was built from scratch around this concept. Firefox wasn't, and there's a whole wealth of infrastructure around the non-multiprocess approach, from the way plugins work, all the way on up. Migrating towards multi-process has been a decidedly non-trivial task for Mozilla, involving a lot of effort from a lot of people.