The justification they've given for removing classic extension support is that they depend too much on the internals of Firefox, for the same reason they also said they're a security risk.
They are valid technical reasons. Most people would agree that making extensions use a stable API decoupled from the browser's internals is a good thing for stability and compatibility in the long run.
But, and this is a very big but, that means many popular current extensions can't just be made to work with the new APIs. Also, the ones that can be adapted will probably need a good amount of work. The result is that many extension developers have said they will abandon their extensions.
Also, since those powerful extensions are one of the reasons many people keep using Firefox that will surely suppose a big hit on their maket share and that's the last thing Firefox needs.
Their stated mission is to fight to keep the web open, if nobody uses their browser they'll have no money and no influence and hence they can't fulfill their mission.
I know this must've been a hard decision to make at Mozilla but I feel it's not the right one.