Firefox has a cool trick that allows you to delete entries from your browser history real quick--I won't comment on why you'd need to do such a thing :)

When you are typing in a URL in the address, past history URLs that match what you are typing are shown in a box under the address bar as shown in this figure:

Past URLs shown when entering an address in the address bar

Once this box shows up, you can use the up and down arrows to scroll through the results, or you can keep typing to further refine the results. Either way, once you find the entry you want to delete, use the arrow keys to highlight then press the Delete key while holding down the Shift key (Shift + Delete some would say). When you do this, the highlighted address will be instantly deleted from your browsing history.