I use firefox (ubuntu) with enlarged fonts and have a problem with LaTeX formulas. They don't fit the space they a meant to occupy and overlay the ordinary text:
It was ok several months ago, but then it seems that some update has happened. I can't remember exactly when the issue appeared first.
UPD1 Is there a workaround at least? It still happens and many tex comments are unreadable.
I don't know if that helps, the similar report for MathOverflow (look for comment by Charles Rezk Mar 24th 2012). I guess he also uses enlarged fonts.
UPD2 The very option that causes the behavior is "minimal font size". Both Chromium and Firefox exhibit it.
UPD3 I've opened an issue on MathJax's bug tracker, hope to find out what causes the problem.