Greasemonkey is a Firefox extension that allows you to write scripts that alter the web pages you visit. You can use it to make a web site more readable or more usable. You can fix rendering bugs that the site owner can’t be bothered to fix themselves. You can alter pages so they work better with assistive technologies that speak a web page out loud or convert it to Braille. You can even automatically retrieve data from other sites to make two sites more interconnected. #
Pretty much, a Firefox user would install the Greasemonkey extension which would then interface with separately installed “user scripts” that the user could either write himself or find in the repository.
The possibilities are mind-boggling for what an end user could do to improve their browsing experience.