Chrome didn't have this until 2018, as the parent link shows. This is not about multi-process architecture. Firefox is < 3 years behind, not 10, not 14.
Site Isolation launched in Chrome in 2018, but the work started in earnest in 2012 -- see the below check-in. The idea in Chrome dated to before the Chrome 1.0 launch; it was the subject of Charlie Reis's PhD dissertation and he interned on Chrome pre-public launch.
Site isolation proved to be the biggest refactor in Chrome's history, and was one of the motivating reasons for the webkit/blink fork. Making site isolation work touched a huge host of features, since handling iframes out of process has a way of making simple things incredibly complicated.
The example I always gave was: imagine how the "find text in page" browser feature would be implemented. With the entire document in-process, it was a simple for loop. With the document and its subframes sharded across multiple processes, it is now a distributed search problem that requires handling of out-of-order results and stitching them into a traversal order. What's more, to achieve Chrome's security goals, you want to avoid introducing functionality that would allow the [presumed-compromised] process of the outer document to query the contents of the inner document via the find in page feature. So you can't simply do this as a peer-to-peer query between the renderer processes; it needs to be coordinated by the main browser process.
I was wrong about the actual security policy, but multi-process is still a big security win.
And not so related to this, but from what I've heard about cracking competitions a few while ago, Firefox was not even included, it was considered too easy. Maybe my sources were just bad.
And I say this as a Firefox user for the last decade or more.
Chrome was a new project, and didn't have to deal with the legacy of being built on top of the same source code as Netscape Navigator. I do not understand why you are trying to make this out to bash Firefox like they aren't as competent by taking ~10 years to implement multi-process browsing after Chrome. Legacy software and patterns are truly painstaking processes to iterate on.
But yes Electrolysis is the initiative that you should have referred to in the original comment.
http://www.scottmccloud.com/googlechrome/
In early-mid 2008, I created a comic book for Google explaining the inner workings of their new open source browser Google Chrome.
If I'm mixing this up with https://wiki.mozilla.org/Electrolysis, that's still 10 years.