No idea what causes it.
@cmpm : Yes, it seems a bit like a "lo-fi" version of the page, but not exactly (from what I saw, lo-fi seems closer to "No style" -- the image I attaches is almost identical to the original website, just without fills, borders, etc.)... I don't think that there's any lo-fi version for this website (DC) anyhow -- or is there one?
@Gothic : nope, I'm not using FF3... Just plain 188.8.131.52
Right now, after restarting firefox (and clearing the personal stuff) everything is OK, and it will probably be for a few hours, or a few days. So I'll have to wait for the phenomenon to happen again before I try the "safe mode" experiment or other tricks. It's not a huge problem anyways, just annoying to not know why it's doing that and won't stop doing it unless I restart firefox etc.