This is something I've spent some time trying to figuring out myself and none of the articles on firebug I've found mention much. The Web Developer Toolbar is the defacto way to edit CSS it seems. But with it, your background images dissapear as soon as you click "edit css"... so it's not my favorite.
Here is a way to copy your css edits made with the inspector that works. The down side is you'll loose all of your comments and formats like manual indents.
First make all of your css edits like normal with the inspect button. Then click the CSS tab (second row). Now choose the css file(s) you edited (probably template.css but you'll have to do some research if you don't understand) from the top row, to the right of the edit button.
Do not click the "EDIT" button. You then copy the css you see there and paste it into your template's css. Hint: if you choose select all (ctrl/cmd A), you will also copy firebug's debug code and all HTML for some strange reason. But if you manually highlight the whole whole thing (no ctrl-A), you'll get what you need.
Choosing the "EDIT" button for css is a whole other way to work. But choose it, and you'll loose all the changes you made using the inspect button. For some reason, it work work together with that (as far as I can tell). But you CAN make on the fly edits, with background images...unlike the web dev toolbar... and then neatly copy it at the end, comments and all!
If anybody knows a way to make it play nicer, I'd love to hear your incites.
I wish I had read that before it happened to me that my background pictures disappeared after making changes in the css file with Webdeveloper and Firebug. It happened to me last night after downloading those plug ins and I hoped them to make things a bit easier on me since I am fairly new to all of this. I was trying to modify a few things in my Virtuemart pages.
Anyway, have cried enough about it
and now I would like to know how I will get my addtocart and notifyme + a few other buttons back. Will I just have to rewrite it in the theme css file?
I was so pissed last night that I even wouldn't look at the css file anymore, what maybe got deleted or changed.
Any help or advice will be greatly appreciated.