It would be great if there was a way to overwrite the built-in CSS for high contrast mode.
Firefox should add it as CSS property: high-contrast
or -moz-high-contrast
(like IE does with -ms-high-contrast
) or give addon developers a way to approach this.
So far users that have to use high contrast mode are just unable to use certain addons - like a text marker.
Some of my addon users addressed it via mail.