so ive been working on a small nodejs project and some of my users recently see weird black bars on it, i have no clue what might cause them and i cannot reproduce them at all.
here is a screenshot: 
their browser version is Firefox 9.0.1
does anyone have an idea what might cause them?
update:
the problem still exists, i made one of the users turn off his javascript and he says that he still sees those black bars. i also made him install firebug and he says that he cant select the bars – only the elements underneath them. (dunno if he did it right though)
its quite strange, the only elements he sees right from the start are the ones which i positioned fixed.
ive been able to reproduce the bug on my laptop and found out what was causing the black bars:
but i still dont know WHY this would happen. i’m using a linear gradient as a background in my header and a heading with multiple text shadows.
maybe this caused the bars on certain graphic cards.