ESSENTIAL Rated 5 out of 5 stars
This is THE classic add-on for FF. Probably one of the add-ons that make FF a power-user's swiss army knife. And the best part? DTA! has stayed true to its roots; hasn't changed much in terms of usability all these years, and I hope it never does!
The only add-on I would almost give a 5 star review, actually I want to give it 4.5 but I can't, so it gets a 4 which is very high in my book.
update: DTA gets the only ever 5 star I've given to date (maybe ever will?) for being tried, tested and true since it first was released. Keep it up!