We are aware of this problem. It is caused by the lack of support for the display: inline-block CSS property / value in that version of FireFox. When FF3 was released, they implemented this mode (which is in fact a standard CSS2 value), which allowed us to drop a couple of vendor-specific (-moz-properties) that we used to emulate this behavior. What you can do is to set the following CSS:
display: block !important;
float: left !important;
However, have in mind that this may affect your layout because of the float, i.e - if you have a <div /> after the button it will stand next to it, not below it. To fix this, use <br style="clear: both;" /> or apply clear: both; to the <div /> itself.
Unfortunately, there are no browser hacks for FF and I cannot give you a solution that will execute only in FF < 3 versions. On the other hand we do not plan reverting to the -moz properties, as they are hard to maintain and cause other problems.
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