click on the down arrow on the search tool click on " Manage Search Engines.." then do a search for Marriam-Webster. once the search result comes up click on the green "add to firefox" button.

The solution given by ammanrex for the question I had asked re: adding Merriam-Webster Online dictionary as a search tool in Firefox, was quite useful and I have added the additional search tool. Also, the answer by ammanrex has shown the general method for adding many other additional search tools as and when needed. Many heartfelt thanks to ammanrex for the prompt and right solution provided for the question I had asked. - - arasan77