The kind of isolation in the post, where the web content doesn't have access to system resources, was part of the original Chrome release.
(You mentioned site isolation, which is a different feature, but the dates you have look right for that.)
(You mentioned site isolation, which is a different feature, but the dates you have look right for that.)
I think you're confusing the general "sandbox that restricts access to the kernel" with the specific mitigation here. Firefox also had the generic kernel mitigations long ago: https://news.ycombinator.com/item?id=31362935
But it's true Chrome had them first. The same thread points out that Firefox reuses Chrome's sandbox, so like in the first sentence, there are causality constraints that would make it hard for Firefox to support it before Chrome did :-)