There are a couple I sorely miss. Disable Ctrl-Q died, and so did Toggle Animated GIFs. Now I have to keep an extra tab open with a warn-on-page-close handler to prevent Ctrl-Q fat-fingering from killing my session. And I've just disabled video/GIF animations entirely, instead of using the cool extension which let me start/stop them on demand.
I also used to have a cool cookie exporter extension, which was useful in combination with wget for scraping sites that required a login. I admit I haven't searched for a replacement, though, so maybe there is one.
Also I'm curious, what extensions are missing? Most of my pre-quantum extensions, such as Tree Style Tabs, have been updated now.