Let's not mix everything up. For a good debate we need clear ideas. Firefox can be built from sources available under open source licenses, it is sufficient to makes it open source. Being embeddable and modular may be desirable features but it isn't a requirement to be open source.
AFAIK, all of it; there are even full independent forks running around. Just because you can't bend the code to your particular use doesn't make it not FOSS.
I know that you meant something else with this, but the wording amused me as currently Firefox is the only up-to-date "portable" browser that lets you carry around your browser profile in a usb stick.
Anything with Chromium deletes your extensions, passwords etc. whenever you open it in a new computer.
But your profile is not fully portable. They have a page explaining exactly what I mean.