Firefox supports international characters for languages such as Hindi. You can test your Firefox's support of Hindi scripts at BBC Hindi. If that site doesn't display the characters properly, information on how to enable complex text support on your system can be found in this Wikipedia article.
Some websites require that you download additional fonts to view their content. Internet Explorer has a feature where these fonts are downloaded automatically; Firefox doesn't have this feature, so you'll have to download the fonts manually.
Most sites that require additional fonts will have a page describing where you can get the font. Look for a link marked "Font help" or similar.