|I assume everything is OK on other windows, such as "Computer" and the pictures are OK.|
In which case it is most weird that this problem only arises in browsers. Neither of them have any settings for colors. You can change colors in Display Properties but these are system wide.
Scraping the barrel I can only think that in some way the two similar arrangements in the browsers either affect your monitor, or more likely your display card.
Assuming the monitor is set up correctly (and that your black is not dark blue) then perhaps it would be worth updating your graphics drivers.
Nobody has joined us so this probably means that there are not many ideas about. Just the same it is worth checking this post from time to time to see if someone else comes up with any bright ideas.
There websites around where you can upload images for free so that we could see your exact problem. You would need some sort of screen capture facility to do this (there are freebies around).
Please come back and let us know the outcome.