Firefox addon to execute Javascript on specified pages?

I'd like to find a Firefox addon that wou开发者_如何学JAVAld look at the url you're viewing, match it against a pattern, and if there was a match it would execute arbitrary Javascript (maybe by loading the .js file from another url).

I can't seem to find any such thing, but it must exist. Anyone seen something like this?

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Check out GreaseMonkey:

https://addons.mozilla.org/en-US/firefox/addon/748/