Firefox’s site isolation uses one process per tab until you have more than four tabs open for the same site; then Firefox makes the tabs for that site share the

Firefox’s site isolation uses one process per tab until you have more than four tabs open for the same site; then Firefox makes the tabs for that site share the four processes. So if you open four youtube.com tabs, you will have four youtube.com processes. If you then open two more youtube.com tabs, you will still have four youtube.com processes, but two of the processes are managing two tabs each and the other two processes are still managing one tab each. This design is to reduce memory usage. I think Chrome uses a separate process for each tab.