Apparently Java mode doesn't work. For solving problems it's not an issue, since there is a javascript mode, but I can't edit problems anymore.
I'm not sure if it's just a problem on my end, but considering nobody else has added a problem in a while, I'm going to assume that I'm not the only one experiencing this.
Java mode seems to have stopped working
Re: Java mode seems to have stopped working
For anyone else wondering: Firefox stopped supporting Java in March. If you want to edit problems, you will have to use Internet Explorer.