A plugin for manipulating JavaScript/HTML code

I need a tool that can parse and insert code to the JavaScript/HTML code before the browser starts to interpret the code. I've been thinking using a proxy to do it. But now I'd like to 开发者_如何学Goknow whether I could implement such functionality in a Firefox plug-in?

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Sounds like Greasemonkey to me.

What does Greasemonkey do?

Greasemonkey lets you add JavaScript code (called "user scripts") to any web page, which will run when its HTML code has loaded. Compared to writing extensions, user scripts often offer a light-weight alternative, requiring no browser restart on user script installation nor removal, and work with the common DOM API familiar to any web developer (with somewhat elevated privileges for doing cross domain XMLHttpRequest requests and storing small portions of private data). User scripts work more or less like bookmarklets automatically invoked for any URLs matching one or more glob patterns.

http://wiki.greasespot.net/FAQ

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I'm pretty sure something like TemperData might work. Or maybe Fiddler, but that's an application with additional hooks that enable it to work with Firefox.

TemperData: https://addons.mozilla.org/en-US/firefox/addon/966/
Fiddler: http://www.fiddler2.com/fiddler2/

Of course both work on a network level, so they may be a bit more arcane than what you'd need.