There I was, developing some Ajaxy/Javascripty front end kinda stuff, and I needed to popup a small div at the exact place that a user clicked. My prototype worked just fine, but in the project code, (where this hting needed to live), my coordinates weren't coming back as I'd expected. I promptly places a few strategic alerts and investigated the values that my function was returning. Nothing out of the ordinary, but if I could only find out if they were accurate..."Where does firefox think the mouse is?"
I search high and low for a tool that I thought was there, it had to exist...just a little something to tell me where firefox was thinking my mouse cursor was. I could've sworn I'd seen it. Finally, I gave up searching and figured I could write one myself, and actually learn something along the way. XUL to the rescue. A few hours later, (after some reading and several conflicting sources of information, I had a working extension.
As a side note, the value I was being given back from my function were indeed correct, but the way our application was modifying the DOM Tree via javascript, etc...the browser was thinking only element on the page was the one in which my click events were being captured...it's weird and still not solved...but anyway, I have this tool now, so if you can use it then please do so...tell your friends, your neighbors, hell put it on Digg, send me money...I'll be rich...FAMOUS. Nah, I just hope it helps you out in some way.
Finally Install Mousetracker