try google indic transliteration Rated 2 out of 5 stars

try https://addons.mozilla.org/en-US/firefox/addon/8731

this uses google indic transliteration.

This review is for a previous version of the add-on (0.4.1). 

Nice

Thanks for the information.

While it supports all the Indic languages that are supported by Google Transliteration, it is not supporting various Tamil keyboard layouts.

And also, the phonetic transliteration done by Google is not better for some words: Try 'guha' or 'guhapriya' in Google Tamil transliteration and you will know what I mean ;-)