[Update: This add-on might not work with newer version of Firefox. Please use Chirpy instead. Chirpy is available for Google Chrome as well.]
Read Wikipedia article on this addon here
Here comes a speaking add-on. Literally, it speaks. Did you ever come across a word which you were not sure how to pronounce? Ever wondered an easy way to hear the word? This add-on is surely going to get a place on your best buddies list. You only need to select (highlight) the word(s) to be pronounced, right click (ctrl+click on mac) and choose "Speak it". That's all. Your new buddy is going to say the word/words for you. See screenshots to put the icon in top toolbar for easy access.
You can even download what you just heard. Yes, an audio file can be downloaded from the same page. Right click on 'download mp3' link, select 'save link as', select location on your computer and it downloads the audio file there. Cheers!!!
Courtesy: fromtexttospeech.com, tts.speech.cs.cmu.edu and vozme.com