Rated 4 out of 5 stars
This is a great add-on to say the least. This add-on allows a user to quickly bookmark a desired link to his or her del.icio.us account without hassle. It is accessible/functional via toolbar, toolbar buttons, and/or right-click menu.
The one aspect of this add-on that could use some improvement, applies only to Linux users (as far as I know). As Firefox is not quite as debugged in Linux as it is in Windows based operating systems, one will occasionally notice a lag in typing response when adding/editing a bookmark to del.icio.us using this addon when there is a heavy or greater-than-normal utilization of system resources. This can occur if there are too many tabs open, etc.
All-in-all, though, del.icio.us buttons 1.2.1 by del.icio.us is a useful, practical, and sensible add-on.