Request filters: These add-ons filter requests to 3rd party hosts, effectively blocking everything (if set to default to deny all). Most sites, other than web applications or ecommerce, only need to connect to at most a single remote host to pull down their CSS files; the next most common requirement is Google Hosted Libraries.

* RequestPolicy: No longer developed, but still works for me

https://addons.mozilla.org/en-US/firefox/addon/requestpolicy...

* RequestPolicy Continued

https://requestpolicycontinued.github.io/

* Policeman: Haven't tried it, but AFAIK it also filters by data type (e.g., allow media requests from x.com to y.com, but not scripts)

https://github.com/futpib/policeman