1.) Find and open the \res\fonts\ folder in your Firefox or Netscape installation (usually located in C:\Program Files\Mozilla Firefox\res\fonts and \Program Files\Netscape\Netscape\res\fonts respectively).
2.) Open the file fontEncoding.properties with a plain text editor such as Notepad, find the line for the symbols definition, replace encoding.symbol.ttf = Adobe-Symbol-Encoding by encoding.symbol.ttf = windows-1252 and save the file.
3.) Also, edit the file ...\res\mathml.css by changing the order of fonts from font-family: CMSY10, Symbol, Times ... to font-family: CMSY10, Times, Symbol ...
4.) Restart Firefox/Netscape.

Please note that this fix can not be applied in Firefox 3 anymore (there is simply no file fontEncoding.properties anymore).