Since Mozilla keeps changing how these relative paths load, I believe your idea of manually specifying an https protocol sounds prudent. Feel free to create a pull request with these changes in enhancedsteam-firefox.css in the dev branch here: https://github.com/jshackles/Enhanced_Steam/tree/dev
@c12h did the work here, I'm just porting over the PR.
This is a port of https://github.com/jshackles/Enhanced_Steam_Firefox/pull/115 into the main Enhanced Steam repo, per @jshackles's comment in jshackles/Enhanced_Steam_Firefox#114:
@c12h did the work here, I'm just porting over the PR.