I have the same problem.
I use a variant of Firefox. (Palemoon. It has the exact same rendering engine as Firefox).
The problem seems to be that the page consists of a black background, with a yellowish square on top, which looks like a piece of yellowish old paper. And there are black letters on top. In Firefox, that yellowish piece of paper is not there. Therefor we only see the black background and the black letters. Unreadable.
Using another browser is not really a solution. Nobody should be expected to use 10 different browsers, just because different websites work only with specific browsers.
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More fun: I can't login in this forum with IE. The same username (email) and password work for Palemoon. But not for IE. Even when I log out in Palemoon, I still can't login in IE. I also tried to edit a post. Doesn't work on Palemoon. I can edit my text, but when I click "save", nothing happens.