Making this style work in FireFox(part2)

<style>
.50{font-size:50px}
</style>

<table>
<tr><td class='50'>abcd</td></tr>
</table>

The font size of "abcd" is 50px in IE6, but isn't in FireFox.

How can I make the font size of "abcd" 50px in FireFox?

I believe that you can't start a class or id with a number.
This works:

<style type="text/css">
.fifty{font-size:50px}
</style>

<table>
<tr><td class="fifty">abcd</td></tr>
</table>