I did a little research and the problem may be with the particular web page you are viewing, not with Firefox. If the author of the page leaves the encoding off the page, then the browser has to guess which encoding it is. The browser may get it wrong. Sounds like you can change your default decoding on your browser to see if that works. Hope this info may give you another road to check out.
It happens randomly with a variety of websites. BTW, the same sites display OK in [shudder] IE.