Spoiler boxes not working

Spoiler boxes on my home computer don’t work right. They are there, but the text is plainly visible, rather than a black box. Is there some setting I could look for, or maybe fonts I need? I don’t have this problem at my work computer.

Both computers run Windows XP, and I’m using Firefox on both (2.0.0.12 at home, not sure at work, but it’s a very recent version.) It’s been like this at home for as long as I remember, so I don’t think it’s a Firefox version issue.

(Maybe this belongs in ATMB instead, but I think it’s my computer, not the board. Although I guess it could be a board setting.)