> I'm guessing the new extension system is a lot less flexible than the old one?

For security reasons, yes.

For people who don't want to say bye to old extensions which are not (or cannot) be ported to 57+ there is the option to stick with 52 ESR version (Extended Support Release). According to Mozilla FAQ [1] this version gets obselete in march 8 by Firefox 59 ESR.

[1] https://www.mozilla.org/en-US/firefox/organizations/faq/