It looks like the development of that extension stopped in 2014 but it's still bundled with Firefox in Ubuntu https://packages.ubuntu.com/search?keywords=xul-ext-ubufox

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.

I hate it when people who know how to disable them don't do it and complain about it. It is really Ubuntu's fault for bundling an addon that is not compatible with multiprocesses.