We haven’t disabled adding other repositories. Like rjm I can trivially add Debian to my sources list.
Regarding add-ons; there are solutions for PureBrowser: Add-on installation from mozilla.org fails
We’ve had long, technical and trademark discussions with Mozilla regarding PureBrowser and Firefox. At the moment our amicable discussions have led us to leave Firefox packaging as Mozilla prefers to have it and we’re focusing on GNOME’s Web browser as our default in PureOS.
Personally I also use Firefox Developer Edition through flatpak. I pull this down from an unofficial repository but I feel comfortable with it, though I don’t use that often. My recommendation is to use the official Firefox from Debian if you want Firefox; it is packaged by Mozilla officially and has Debian’s security support.