I tried CSSVista, a free application, but I didn't really understood how it works.

I finally found Internet Explorer Developer Toolbar made by ... Microsoft itself. Well, it's not firebug, but it does it's job.

Internet dev toolbar

Note that the html code is rewritten. For example :

<div class="foo"><p style="border: 1px solid black">bar</p></div>

will be show in the toolbar as :

<DIV class=foo>
    [-] <P>
        [-] #text

It looks like that old HTML 3.2 or whatever, without quote, in uppercase :(