Can you provide more information specifically about your email, rather than Firefox? As in, what email client do you use? If you aren't using an email client, are you accessing your email right from the web? ---It sound like you may be using the "webmail" version of your email, based on your comment about being unable to sign out.

Asking for more information because it may be the email service's website, and not your browser, or email client, that is causing the problem.

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.

Make sure that you allow pages to choose their colors and that you haven't enabled High Contrast in the Accessibility settings.

• Tools > Options > Content : Fonts & Colors > Colors : [X] "Allow pages to choose their own colors, instead of my selections above"

• http://kb.mozillazine.org/Website_colors_are_wrong