The problem is that when you get more and more tabs going, Firefox's single-threadedness becomes more and more painful. When one misbehaving tab locks up (or crashes) the whole browser, that's bad.
That could explain why it doesn't support multiprocessing. I disabled it because it doesn't do much. This is a list of its functionality from https://ubuntu-mate.community/t/what-are-the-ubuntu-firefox-...
* Enable searching for missing plugins from Ubuntu software catalog
* Add the following options to the Help menu
Get help on-line
Help translating Firefox
Ubuntu Release Notes
* Set homepage to Ubuntu Start Page
* Display a restart notification after upgrading Firefox
* Add ask.com to the search engines. You can uninstall this if you prefer to use a pristine Firefox install.
That's why I switched back from Chrome after using it for a month few years ago.
Now I would switch because Chrome is the new IE, some developers don't test on Firefox, they say "just use Chrome", no WAY.
I find it uses probably 50% of the memory that Chrome does in my typical use-cases as well (4-5 windows open with around 10-20 tabs in each).