This has been considered and rejected as inferior to the current approach. The Android browser used this and it was rewritten to get rid of it.
Let's focus on doing things that improve the user experience rather than blindly copying features from other browsers without considering if they make sense.
Moreover, it has absolutely nothing to do with the question asked.
Plugins and add-ons AFAIK. There are also memory/performance implications, which apparently were not insignificant on mobile platforms. Given that Chrome uses more memory than Firefox, it's perhaps not insignificant on desktop either.
This has been considered and rejected as inferior to the current approach. The Android browser used this and it was rewritten to get rid of it.
Let's focus on doing things that improve the user experience rather than blindly copying features from other browsers without considering if they make sense.
Moreover, it has absolutely nothing to do with the question asked.