Firefox won't have a process per tab. Even in the multiprocess versions, it has one process for the UI and one for all the tabs' content. That way it doesn't take tons of RAM for overhead if you have lots of tabs, but heavy tab content can't freeze the UI. You can change the number of tab-processes in about:config under dom.ipc.processCount.
Another advantage of using one process for all tabs is that Firefox can take advantage of security sandboxing of the content process without the overhead of tons of per-tab processes. (Quite a few Firefox users like using hundreds of tabs. :) Eventually Firefox will tweak the tabs/process balance to find a good compromise.
Firefox uses compartments per domain instead, which is a great alternative to process per tab:
"Some readers might wonder how compartments compare to per-tab processes as they are used by Google Chrome and Internet Explorer.
Compartments are similar in many ways, but also very different. Both processes and compartments shield JavaScript objects against each other.
The most important distinction is that processes offer a stronger separation enforced by the processor hardware, while compartments offer a pure software guarantee. However, on the upside compartments allow much more efficient cross compartment communication that processes code.
With compartments cross origin websites can still communicate with each other with a small overhead (governed by certain cross origin access policy), while with processes cross-process JavaScript object access is either impossible or extremely expensive.
In a modern browser you will likely see both forms of separation being applied. Two web sites that never have to talk to each other can live in separate processes, while cross origin websites that do want to communicate can use compartments to enhance security and performance."
Yeah, they have a MemShrink team now. https://wiki.mozilla.org/Performance/MemShrink They got serious about memory leaks in their own code, and even took steps to limit hogging by 3rd-party plugins. Now they host https://AreWeSlimYet.com/ to track regressions. Edit: and Chrome really does take a ton of RAM if you have lots of tabs open.
"and Chrome really does take a ton of RAM if you have lots of tabs open."
Yup, it does. My problem with FF years ago was with leaks. It would eventually bring my entire system to a state of paging hell until I killed it. That was years ago though, I've since become comfortable with chrome and haven't found aneed to go back.