How can I patch client side javascript on a website after loading it in Opera or Firefox?

Question

There is a particular website I must use for work which is absolutely heinous and despised by all who must use it. In particular, the site's Javascript is fundamentally broken and works only in IE, which pretty much makes it the only site I must use outside my preferred browsers.

So, to the question. If I could 'patch' the javascript after loading the website in such a fashion as to 'do the right thing', I could then use the website without IE.

( Just to cut out some of the superfluous answers: I have already tried masking both browsers as IE, which has no effect because the issue is with the javascript, not browser detection on the server. )

I would prefer solutions which are for Opera, though I'm not opposed to Firefox answers. Also, I would rather not have to view the site though a proxy, though I will entertain such answers.

Answer 1

For Opera, you want User JavaScript. Similar to Greasemonkey, but built-in to Opera. Built to be used for exactly the sort of situation you're in: fixing sites that are broken in Opera...