How does Firefox know which of the 4 processes owns which ta

[fsckDsk]

Firefox newly started using multiprocessing similar to Google Chrome, except all tabs are split to four processes instead of one process per tab.

How does Firefox determine which tab goes to which process?

[dickvl]

Firefox uses one process for the main Firefox thread (user interface)
There can be one or more content processes set via Options -> General -> Performance
You need to remove checkmark to see the current performance settings: "Use recommended performance settings"
On Windows there is an additional process for the compositor thread (graphics) to make it possible to restart this process in case of issues with the graphics display driver.

You can find the current multi-process state on the Troubleshooting Information page.
Help -> Troubleshooting Information -> Application Basics: Multiprocess Windows and Web Content Processes

See also:
https://support.mozilla.org/kb/performance-settings

[jscher2000]

How does Firefox determine which tab goes to which process?

Mozilla has a bunch of wiki pages where you might be able to find this information. The code name for multiprocess was Electrolysis, so some of the pages may be under that.

[MarkRH]

Doing a Measure at about:memory and going to about:performance and expanding out a site (clicking "more") will give detail on what Process ID is doing what.

[makaiguy]

Please educate me. I'm content knowing FFox is using my system's multi-processor capabilities. Why would want to know which processor is doing what?

[Brummelchen]

Firefox newly started

electrolysis starts long ago
https://wiki.mozilla.org/Electrolysis
was "on" since firefox 48 if the requirements were met (e10s compatible extensions).
and firefox 48 was released around 01-Aug-2016 - this is now 2 years ago - new?

Why would want to know which processor is doing what?

for me its just a footnote - chromium builds have taskmanager - for my nosiness.
firefox its capabilities to control tasks and to recover them and pages and more is working well. while main thread is still working.