Sorry, perhaps I misunderstood the tagging process. I just tagged it as Hindi because I thought it might help people searching for this problem. I'm not using a Hindi install of OS X nor a Hindi localization of Thunderbird. I shall also take off the Hindi tag now. Sorry for the confusion!

I don't think so. I encounter no problems in Firefox while viewing multiple scripts and languages, including that same "Devanagari MT" font, which does come built into OS X. To me, the problem seems to be that I can't actually select the Devanagari MT font in the drop down list in Thunderbird, where one chooses the fonts in which to display certain, simply because the font doesn't show up in that list. It does, on the other hand, show up in that same drop down list in Firefox, which is probably why it works fine there.

Also, as I mentioned in my original post, if I copy the question marks displayed in the message from Thunderbird to something like TextEdit, the message displays perfectly fine. This also leads me to conclude that I don't need a Hindi localization for OS X or Thunderbird.

To summarize, what is frustrating me is that it works in Firefox, works in TextEdit, works in Mail, Safari and iWork, so why not Thunderbird?