I'm currently using Firefox 4.0 on OS X, with STIX fonts installed, and for some reason MathJax (in HTML-CSS mode) is not rendering the \not symbol at all, i.e. for me $a \not = b$ looks like $a = b$ rather than $a \ne b$. This rendering problem seems to be limited to StackExchange; I don't have any problems on MathOverflow. Chrome and Safari also render \not correctly.
We do not, as a matter of policy, support beta browsers. Feel free to ping me on this answer if the problem persistes after FF4 is released.