MathJax is not working in 1.23wmf22 and later, incl master.
1.23wmf22 is currently deployed to group0 and will be deployed to group1 and group2 on tuesday and thursday respectively.
The breakage can be witnessed here:
1: There seems to be a change in how the mathjax setting is kept, but users do not appear to get migrated into the new setting. This is bound to create community friction.
2: In PNG mode, the mathjax option doesn't seem to work at all
3: in Tex mode on betalabs I get:
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (MathMenu.js, line 0)
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (MathZoom.js, line 0)
Suspected cause of breakage is Bug 57981, commit https://gerrit.wikimedia.org/r/#/c/110226/