A major, trusted dictionary that describes language rather than prescribing it (like the Oxford English Dictionary) does, webster's dictionary is authoritative without attempting to control the English language according to the wishes of elite, conservative lexicographers.
This add-on takes me straight to the page of the definition, saving me two pages: the search page and the half-a-definition page that I think contains ads I've blocked. Therefore, it saves me considerable time and bandwidth on mobile Internet.
The pages are faster to load and freer of junk than thoseof Dictionary.com, but could be cleaner: it would be nice to have less 'Did You Know?'-style rubbish and less advertisement -- but it's not too bad. I'm guessing the paid service is probably cleaner, more comprehensive and more configurable -- there's no such thing as a free lunch, or not often, anyway.
This lunch comes with no added poisonous propaganda (to my knowledge), minimal packaging and high nutrition.
To rebut a couple of other reviews: 'American English, not English' -- I'm guessing it lists British English usages too. And many users will be American -- death to pretentious xenophobic snobbery.
'Dictionary.com is better' -- it's bloated and I think the Webster's in that is the 1913 one. Dictionary.com has many dictionaries, some of which are not trustworthy. I'd rather get results straight from the dictionary I trust without unreliable or outdated garbage to sift through.
Doesn't work with Ubuntu? It's working with Iceweasel 220.127.116.11 on Debian 7. :->
Takes you to an ad page? It doesn't do that to me.