It seems the way it's supposed to work is by entering the following into the address bar:
javascript:(function(){if(window.MathJax===undefined){var%20script%20=%20document.createElement("script");script.type%20=%20"text/javascript";script.src%20=%20"https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS_HTML";var%20config%20=%20%27MathJax.Hub.Config({%27%20+%20%27extensions:%20["tex2jax.js"],%27%20+%20%27tex2jax:%20{%20inlineMath:%20[["$","$"],["\\\\\\\\\\\\(","\\\\\\\\\\\\)"]],%20displayMath:%20[["$$","$$"],["\\\\[","\\\\]"]],%20processEscapes:%20true%20},%27%20+%20…