Longman Dictionary, my favorite one, offers online dictionary service through its website Longman Dictionary Online http://pewebdic2.cw.idm.fr/.
I did not notice any intent to offer integration tools, instead they only made available pronouncation .wav files for words starting with two letters, but luckily it is still quite easy to integrate Firefox with the website through a search plugin (if installed manually, it should be copied into %user%\Application Data\Mozilla\Firefox\Profiles\%profile%\searchplugins).
Make sure to download an updated version – http://alax.info/blog/606