I meant doing so without adding it to every search. But, hey, I clearly didn't have enough faith in humanity... not only am I now able to find some extensions to generally mess with my search queries, there's extensions freakin' DEDICATED to filtering out W3Schools!!!

https://addons.mozilla.org/en-US/firefox/search/?q=w3schools

(Edit: that doesn't mean I necessarily trust or recommend niche extensions like these.)