Firefox supports nearly the same web-extensions API as Chrome. Often porting to firefox means a couple of tweaks are necessary on the part of the author, but try https://addons.mozilla.org/en-US/firefox/addon/chrome-store-... and see if maybe the extension you want is already usable.
From a developer perspective, I'd say bit modern as well. For example, FF extension APIs use Promises, which are relatively pleasant to deal with than callbacks that Chrome uses.
