Okay, so according to this thread in the Mozillazine Forums, the problem is this:
Previous versions of FF did not adhere to character encoding standards, and when a particular character was used that was not available in the specified or expected available character set, it tried the next on the list until it found the right one. Thus characters displayed correctly through an automatic process of best-guessing.
FF3 now adheres to the standards. It displays the page in the character set specified by the web page, or the server, and does not guess. Instead, when you encounter an absence of character, you have to specify it yourself via the View menu, or add the appropriate International Font to your OS.
The argument is, it’s not FF3 that’s at fault, it’s the web pages and servers not specifying the correct character set that was subsequently used in the page.
I can reluctantly accept that. But I think it’s a poor way to “fix” something that wasn’t, essentially, broken, and it needs to be addressed, by Firefox, in a better way.