Firefox windows do not refresh properly when overlapping other windows, occasionally blocking view of others.
Whether with other Firefox windows or other application's windows, the area of overlap will be obscured by a Firefox window behind the active window. This appears to be a screen refresh problem.
Whether with other Firefox windows or other application's windows, the area of overlap will be obscured by a Firefox window behind the active window. This appears to be a screen refresh problem.
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Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.
You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).
orange Firefox button or classic Tools menu > Options > Advanced
On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"