This is really interesting. Prior to this, Firefox's isolation model was much weaker than Chrome's due to only having a pool of 8 content processes. If I'm reading the technical blog correctly [1], this will move to a process-per-site model without also doing process-per-tab as Chrome does, i.e. if you have several tabs open on the same site, they'd be in the same process. This seems much less resource intensive than…
This is a common misconception. Chrome doesn't technically do process-per-tab.
Chrome's model can most succinctly be described as process-per-domain, although even then, there are rare instances where two tabs opened on different domains will actually share the same process.