Is Firefox 57 multithreaded?

According to Multiprocess Firefox on MDN, Firefox 57 finally runs things in different processes.

So is Firefox 57 multithreaded?

In particular, how many cores does it use at most? Say I open 20 tabs at once on a quad-core machine, will Firefox 57 launch as many threads, or at least as many as needed to max out my CPU?

(Chromium for instance spawns a number of sub-processes and very easily uses all the resources available. Firefox up to 56 in GNU/Linux is rarely seen going over 135 % of one core however many tabs I open.)

Solution 1:

Is Firefox 57 multithreaded?

Yes, and it is configurable as well.

You may see 3 or more processes running in the Task Manager

• one process for the main Firefox window (user interface)
• one or more processes for content threads set via "Options/Preferences -> General -> Performance" (dom.ipc.processCount), default in 57 is 4.
• one process for the compositor thread (Windows)
• one process for Quantum CSS (Stylo)

Source Why is Firefox Quantum running 7 instances?

Further Reading

• Change how many processes multi-process Firefox uses
• "E10S-Multi:" A new multi-process model for Firefox