I'll take care of it.
I know you've already pushed some bigger changes beyond 1.5.1, so I won't be able to use CI to deploy something on the order of a 1.5.2, but that's fine. This has been broken on Windows since July of last year and this is the first report we've gotten on it! So I don't think there's a huge need. We can definitely hold off until Sigil 1.6.
I'll make a patch for the 1.5.1 source for anyone who wants to roll their own, and can make an installer available for @snarkophilus and the few others who might not want to live without the fix.
This one's on me. I clearly only tested MathJax functionality on my main Linux machine since version 1.2.0, and didn't look any further when it appeared to work.
Also, I can't seem to recreate the issue I was having on Linux where setting a custom Sigil temp folder was breaking MathJax functionality in 1.5.1. Perhaps I just missed a restart after changing the enable javascript setting