Do you want to delete one item from your browsing history? Say that splash page, or misspelled site that you keep visiting when ever you type a url with a similar name because you type to quickly? Gogle instead of Google? It happens. Just select it by entering the partial name in the url bar, and press shift delete. That will remove only that one entry from history.
Want to remove that memory of the time you forgot to put on pants before you left the house? Sorry, that feature hasn’t been implemented in Firefox just yet. Perhaps in the next version. Patches welcome.