Currently I'm for instance trying to integrate a small speech bubble on my blog profile (see link "What am I doing?" on the left) for displaying the last twitter entry. It already works more or less satisfiable on Firefox and Safari, but it doesn't on IE.
At least for Firefox there exists a nice "must-have" tool called Firebug. You have to check that out, it's great. However most of the problems usually lie on the IE side, where you don't have such powerful debugging tools in place. So since during the last months I just hacked in new changes such that they worked on Firefox, my site doesn't get rendered that nicely on IE (I'm sorry for that) :( . I'll see to fix that as soon as I'm able to.