When you decide to fork a project, a key concern is: how invasive are your changes going to be? This is a very large component of how easy or hard it will be to merge upstream changes in the future.

In this case, going with Chrome would mean very invade changes to enable exposing things the upstream specifically wants to keep private in their codebase. Firefox instead requires only well-delineated, minimally invasive changes because there is a nice clean boundary between UI code and engine code (which incidentally matches c++/js boundary, the languages themselves don’t matter much), which means their changes can be isolated to the UI layer, which in turn means a much cleaner path to merging upstream changes