I have seen firefox have a substantial delay the first time a web page
tries to access a unicode code point (presumably it is collecting data
on fonts and the characters they have available, but that is just a
guess on my part). MathJax uses the unicode font positions quite
heavily, and so the first time it requests one probably causes Firefox
to go through this font-data-collection exercise. I don't know if it
locks up the whole system or not, but it does take a few seconds (and
is probably disk and cpu intensive) so it wouldn't surprise me if it
did.
Does this happen for you on EVERY page, or just the first one (that
uses MathJax) after you restart Firefox?
Davide