http://second-cup-of-coffee.com/tweaking-the-inline-style-of-an-element-in-firebug/
I don’t know why I never tried this, but I finally figured out that simply right-clicking in Firebug’s style window gives you the option to “Edit Element Style…” Yes, it is that simple. Choosing that option will put an element.style option right at the top, just like in Chrome’s Web Inspector.