Nice work, the Dropbox integration is probably a good idea (although I personally don't use Dropbox). One question though: is it not possible to click to the next question, when you're reviewing? Does this require signing in? Coincidentally, I also recently used React to build a SRS site specifically targeted toward review of computer science topics. React definitely makes building UIs into a pleasure. (Now, if only…
Humm I used Mathjax at some point before and will probably go back soon since KaTeX doesn't seem to be updated anymore and Mathjax just got a new very fast beta version. I'll see how well it plays with React then. I would hate to go to pre-rendered images though. That's so cumbersome and just gah :D.
I also used Firebase for Ginseng before! But then no one has a Firebase account to use on their own, and using a central one would mean user management/registration, traffic costs etc. That's why I pulled out. So you're using one account for the program?