Shabdkosh : Select any text on browser -> right click on text -> select 'Find hindi meaning of `Word` with '. There are 3 options for users to translate any word into hindi with (1) http://translate.google.com, (2) http://shabdkosh.com, (3) http://dict.hinkhoj.com. these websites are very fast and reliable.
shabdkosh is very good and reliable Firefox extension designed to translate any english word into hindi with the help of 3 well known websites.
This is a restartless addon, which means you won't be required to restart the browser in order to enjoy its functionality.