bitmap corruptions in Firefox after ~60s of inactivity

Firefox 1.5, every VM, every bit depth.
Start up FF and don't move the mouse for approx one minute.
After that time and a redraw (right click for menu or invalidate the window in any way) certain bitmap types are recolored or display random colors & patterns. Same for the firefox bitmap buttons the navigation bar.
Could be related to some FF redraw timer in connection with alphablend functions (not properly tested yet).