In the absence of CSS rules on an input element (e.g. a textarea) to the contrary, all input elements have a foreground color of black. The correct color is the system foreground color. If the system background color is dark, this makes almost all web input areas (including e.g. this bug report form) illegible.
This is possibly related to bug 107295. See also screen shot attached to that bug report for an example of the problem.