Right click in a free space on your desktop.
Left click on Personalization.
There are four options at the bottom of the screen, choose "Window Color and Appearance".
Select a border color
Uncheck: "Enable transparency"
Re: when i open a pdf it is transparent and i can see my desktop wallpaper behind it making it hard to read
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do NOT click the Reset button on the Safe mode start window or otherwise make changes.