Firefox Options are accessed via the "Triple Pancake Stack" button toward the upper right side of the Firefox Window. Choose Options, then the Advanced tab at the side of the Options dialog, then the General Tab at the top of the Advanced dialog. You'll find the checkbox there.
For what it's worth, one of the things I do is working with individuals who are visually-impaired or blind and who need to relearn how to use their computers with minimal or no sight. I have come to hate the high contrast themes because they cause some very "interesting" behaviors such as the one you describe. You are far better off starting with a standard Windows theme of your choice and customizing it such that the contrast of text on background, and other things, best suits your visual profile.
P.S.: If the websites under discussion are not of a private nature, posting the links to a couple of them, and noting what you can see using one browser versus what doesn't get loaded with another would be helpful.
Edited by britechguy, 10 December 2015 - 02:02 PM.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story