is there a way to prevent this from happening?

You know I am really not sure why this is occurring.
We will have to investigate.

Which Fast Dial Version are you using ?
Are you using a Background from this site ?
What is the name of the background file ?

Have you named your Theme setup (with background loaded) in Preferences/Appearance/Theme ?

Are you are loading your background into Preferences/Appearance/Page/Image ?

In Firefox if you go to Tools/Options/Content/Colors have you got "allow pages to show their own colors" enabled ?

Many thanks

No, the background is wallpaper I found elsewhere.
darkwood.jpg

This one ?

I do not know the cause of your problem, but I will try to reproduce.

Is your background centred or not centred ?

Please check that folders cache and themes are not set to read only. These folders are in the Fast Dial Folder, in the Firefox Profile

My suggestion for the moment, which may seem a little basic, is to try uninstalling and re-installing Fast Dial, and then restart Firefox.

BTW Have loaded your background and it seems to work fine.(2.23b1)

What we could try on your machine, is to load only Fast Dial into a new Firefox profile, and see if the background loads OK.

If that works then we can try and narrow down the cause of the problem in your present Profile.

Many thanks