O Brave New World...

Firefox on the Mac just doesn't support Devanagari, for whatever reason. As of version 3.x I enjoy Safari much more than Firefox anyway (Firefox is sluggish and doesn't support native UI widgets), but I'm hooked on the many add-ons available for Firefox so still use it for most browsing.

By the way, the way you do conjuncts with the Devanagari-QWERTY input scheme is by typing an F (lower case; sorry, I told you the wrong thing earlier) after the first consonant, then the second consonant. So to do ल्ल you'd type l-f-l. (Capital F is how you get the dot under the preceding character.)