EDIT: disregard everything below, apparently I'm the one who fucked something up
EDIT2: for those who encounter the same problem in the future (on Firefox at least): it turns out I enabled
network.http.referer.spoofSource, which made the server return a wrong Location for reloading the page after upvoting. So I just disabled that and all is well now.
...kind of works, but returns a 405 error page because clicking the upvote button sends a GET request instead of the expected POST. Not the end of the world, nevertheless I find it quite annoying.
To fix this, you could either redirect GET requests back to the page where the user upvoted a post, or use a form and return the page where the user upvoted a post.