I was confused about it as well.
https://arstechnica.com/information-technology/2017/06/firefox-multiple-content-processes/
In theory, moving to multiple content processes will improve stability and performance (one bad tab won't slow down the rest of your computer).
You can disable the use of multi-processes, but it might break other things:
https://silicophilic.com/disable-firefox-multiple-processes-task-manager/