MathML has over 2000 predefined entities, many of which are associated to various symbols used in mathematics and science. Recently, assignments for these symbols were approved by the Unicode Consortium. Mozilla can display any of these symbols provided suitable Unicode fonts are installed.
Mozilla is internally Unicode-based. Furthermore, in accordance with the W3C CSS2 recommendation on fonts, authors can specify an ordered list of particular fonts which they prefer (using the font-family property of CSS), with the assurance that Mozilla's font engine will hunt for alternate fonts whenever their specified fonts are not found on a particular user' system. If no appropriate font is ultimately found for a given character, Mozilla will instead display a box containing the hexadecimal representation of the Unicode code point for the character.
It follows that, on two similar systems, while one user may see a document rendered correctly, the other user may see something completely different. To see MathML as intended, you need sufficient font support, which may mean installing some fonts. Just having a MathML-enabled browser is not necessarily enough.