According to some Mozilla people, this part is actually wrong and extension support is indeed not implemented completely. It's merely that I couldn't (and still cannot) find a list of outstanding issues, and everything I tried worked correctly.
They don't. Mozilla at this stage is still a very idealistically motivated organization. Most people are trying hard to do the right thing. It's merely that real-life constrains often lead to rather suboptimal results, despite all the good intentions.
They do lie. Mozilla claimed Quantum was fundamentally incompatible with XUL extensions, despite the fact people could simply build their own Firefox Quantum from source with XUL support turned back on.
