As of Quantum, the only extensions supported are those created with WebExtensions-the older ones don't work. Several years ago, Mozilla introduced WebExtensions, a programming interface for developers that limited extensions, but still gave them some juice. A bad extension could bring the whole browser to its digital knees-a bad look, security-wise. That's because the old extensions had too much power.
Quantum also pretty much killed off Firefox extensions as we know them. Firefox Quantum offered a new and improved interface, faster page loading, better use of memory, integrated screen shots, and story saves via Pocket, which Mozilla acquired in 2017. Last year, Mozilla gave Firefox the upgrade it deserved.