It appears that the MO TeX does not understand macros \mod and \pmod any more; it used not to be the case. As a result for many questions the typesetting is broken.
MathJax v2.6 was released on Dec 30, close to the OP. These updates can lead to errors if a browser cache contains a mix of MathJax components with different versions. In that case, clearing the browser cache from all things MathJax (and for some browsers, additionally restarting the browser) should help.
Seems OK to me...
$5 = 7 \pmod2$
$5 \equiv 7 \mod 2$
Perhaps Dima should tell us what OS and browser versions he is using.