I agree that Firebug is a fairly complex tool with many options - but the only one you really need to look at is the “inspect element” option, the use of which was explained carefully and in detail in the posts @ronpat linked to. That part is very straightforward to use and shouldn’t cause you any difficulties. It’s invaluable for answering questions like this one, or this one. (It’s how I found the solution there in the space of a few seconds.)
You really need to take time to play about with this part of Firebug and see how it works. It will not only show you the CSS rules associated with an element (and mark any which are being over-ridden by other rules), but it will allow you to temporarily change values, delete or add rules, so you can immediately see the effect on your page.
As you know, members here are very willing to help, but we do expect you to try to help yourself, too. You’ve been introduced to a useful tool, and shown how to use it. Now it’s up to you to make use of that information. If you get stuck interpreting results, by all means post here for help, but at least try it for yourself first.