One of most common event related questions I got is about error in Mozilla browsers “event is not defined”.
IE and Mozilla treat event variable differently, while in IE it is a global variable (accessible without passing it to the function) in Mozilla you need to pass it every time (which btw is by w3c standards).

Example [...]

I’ve just finished writing up some docs on the new Cross-Site XMLHttpRequest feature in Firefox 3. I was a little worried at first, but it definitely appears to be both easy-to-implement and easy-to-use. Specifically, it’s an implementation of the W3C Access Control working draft (which is respected by Firefox’s XMLHttpRequest).

If you’re interested in giving it [...]