If you find that colours in Firefox do not match the colours in a graphics editing application such as Fireworks it may be due to Firefox having colour correction enabled.
Recent versions of Firefox have had colour correction turned ON by default.
Deactivate Colour Correction
To deactivate colour correction, go to type about:config into the address bar, find
and assign a value of 0 (zero).
For existing installations where colour correction had already been deactivated, you may find that it becomes activated by default after an upgrade.