Download and install MathPlayer.
- Firefox 2.x, Mozilla, Seamonkey: Download and install this
Font Pack.
If you don't install the fonts, then you'll get equations that look like this:
. - Firefox 3.x: Download and install the STIX fonts. More information about this issue can be found here.
The latest version (9.63, as of this update) natively supports most MathML. You shouldn't have to do anything special to read most MathML pages (there are a few items that Opera will not render correctly).