When ever you search some thing in a search engine text box, or when ever you type the addresses of the various websites in the address bar, Mozilla Firefox saves these addresses and searches so that when you search or type them the next time in the address bar, they appear as a drop down list. This is of good help since you need not type the addresses all over again, instead you can select one from the drop down list.
But when ever you have typed a wrong address or search, or if you want to delete a particular address from showing in the drop down list you can delete that entry from the drop down list.
Just type the words similar to the wrong entry or the entry you want to delete. When the wrong entry comes in the drop down list, just highlight it (do not click it) and then press the delete key (in the keyboard).
In case you are using a MAC you might have to press Shift + Delete keys.