[pmontra]

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.