I had a longer rant to write. Since I don't want to have an excessively negative blog, I'll boil it down:
I am unhappy with Firefox because I can't find a reliable way to get mouse positions with it. window.event.clientX gives me the x-position in both Safari and Opera, but window.event appears to be undefined in Firefox.
As a side note, I have not tried this with Internet Explorer yet. The more I read quirksmode.org the less I am angry at IE. They are terrible in terms of CSS, but it seems that in Javascript they are doing mostly OK. I haven't even startedXMLHttpRequest yet, which is supposed to be hellish cross-browser, but like I said, for now IE only bugs me when it comes to CSS. Sorry, Firefox, you are the weakest link (for now).