In http://www.mozilla.org/projects/mathml/demo/extras.html, "Mixing with other markups", there is a rotating rotation matrix. But the theta angles do not rotate the same as the rest of the matrix. I noticed this bug when I upgraded to Firefox 14.
If I replace e.g. one theta by "t", or one <mi>θ</mi> by <mtext>θ</mtext>, I no longer see this bug.
Could you have a Firefox built with --enable-system-cairo? (Bug 583035)
Yes, I have the --enable-system-cairo option.