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AshishRanjan, you can use your DICT server by assigning a shortcut to your local DICT client and then upon selecting a word (and this works outside Firefox also) you just press the shortcut and your local client should display the definition of the selected word. If your client doesn't work as I just described, try kdict, it should work just like that. Don't know about any other client.
Regarding creating the shortcut for launching the client application, I assume you use GNOME and I use KDE so you might need to seek help on ubuntu forums if you don't know how to do it.