In 2004, the website All Music Guide did a horrible redesign. I was so irritated that I created a Firefox extension that did only one thing — it made All Music Guide a bit more useable. To my knowledge, this was the first "site-specific" browser extension, so I wrote a blog post about it: https://www.holovaty.com/writing/all-music-guide/ Aaron Boodman saw my post and decided to generalize the idea; he created Greasem…
The way it would block ads is to analyze the files it was fetching for the browser and modifying found keywords, for a crude example all IMG tags with SRC containing the string ad.* would be replaced by gray images.
I also remember using it to "fix" a redesign of a site.