I've been working on learning Japanese recently (like, half a week.) and I set my computer to left alt + Shift to change my keyboard layout from Standard English to Romaji to Hirigana translation (if it worked in Firefox, the basis of this question, something like "Boku ga neko desu" would change to "ぼくがねこです")
Anyone know how to change it so it works in all programs that support Japanese input (I'd like to get it working mainly in Pidgin and FireFox, but anything else is a bonus)