...Well, you'd think the fix to such a widespread problem would be easier to find, but NOOOOO! I had to dig through dozens of bugzilla logs before I finally found the solution. Since I did all that digging, I figured I'd share it with the rest of you, so hopefully you won't have to. If you have this problem and want it fixed. You know....uhm, yeah. ONTO THE FIX!
Step 1: Go to Firefox > Preferences > Content > Fonts&Colors > Advanced.
Step 2: In the drop-down box next to "Fonts for:", select Japanese.
Step 3: Under "Serif" and "Sans-Serif", change the font to "Osaka".
Step 4: OK your way out of all the dialog boxes.
That's it! You're done! It should work now.
If this doesn't work...well, I'm not sure how much I can do, since I don't have a Mac, but I can try and troubleshoot to the best of my abilities. So...go ahead and leave a comment. Even if it does work, too, so that I know it worked for someone besides smashfantastic. Enjoy the fix!