Unless the page is transparent and showing something behind the Firefox window, this may be a feature/setting of your webmail site. If you mention which one it is, perhaps a volunteer has an account there and can take a quick look. Otherwise, you may need to contact the webmail site's support.

Hello,

1. Put your mouse at the bottom left of your window (you may notice a tiny icon for picture) and let it hover there for a moment. A menu of backgrounds will appear down at the bottom and your window will go blank. Click on one of the basic colour tiles. That should make it easier to read.

2. To sort by date, sender, subject, etc. Click on 'View' on the top right tool bar of your mail. You should see the option you want. See the attached screenshot with the big orange arrow.

As to sorting, at the top of the mail listings, you can see column identifiers. Press date, the mail is sorted by date. If you press date again, the sorting is now reversed. It is the same with the name other other columns.