The STIX Fonts Project has been developing a comprehensive set of fonts that cover all the symbols in MathML. A copy of these fonts is provided on this wiki and should be installed for best MathML rendering. You should download a Zip archive of STIX fonts 1.0 + additional fonts from STIX Beta. Indeed, even if STIX fonts 1.0 are already installed on your system, you need the STIXSiz*.odf fonts provided in this archive.
Resetting Old Setup
If users have previously changed the "font.mathfont-family" preference for a previous version of Mozilla, then it is best to reset this to the default value. To do this, enter the URL "about:config", "Filter" for "mathfont", and "Reset" to the default value through the context menu on the preference.
It may be best to uninstall BaKoMa TrueType Computer Modern fonts if these have been installed (perhaps for a previous version of Mozilla) because these fonts use a character encoding that is inconsistent with Unicode recommendations. These fonts are not normally used, and thus may not cause problems, but if a document explicitly requests font-family "CMSY10" or "CMEX10" or similar, and the BaKoMa fonts are installed, then they will get used, which will likely result in the wrong symbols being shown.