I think Web Developer is a little easier than Firebug to operate.
Go to the page in question and hit Ctrl-Shift-Y and when you hover over the part of the page you want to know about, the CSS will appear in the address-like box at the top of the browser.
Clicking when in this mode will show the CSS of that element in a separate box (if it's included in the stylesheet).
Ctrl-Shift-E will open a stylesheet editor you can use to edit the CSS and see the results in real time without making permanent changes.
In the case of attributes, the CSS isn't automatically added to the stylesheet, so you'll have to add the statement yourself (for exapmle, #attrib -3) and style away.