Chrome's first public release, which included multiprocess tabs, was on December 11, 2008. There has been more than enough time for Mozilla to catch up.
Note the words "first public release". Not to knock Chrome's achievements in this area, but it's an inescapable fact that they did not have any previous architecture to migrate from. Taking any nontrivial program that has had years to assume a single-process architecture, and updating it to work in a multiprocess manner, is hard. It takes time to do properly.
