Do you have like a screenshot, or example webpage?

First, try to reset Firefox. Refresh Firefox - reset add-ons and settings. This simple step will eliminate many Firefox problems with only one click.

A few other basic trouble shooting things you can try is this: Run all Windows Updates, install all needed service packs, etc.

Update your graphics driver (Firefox uses your graphics card for some rendering, and an out of date graphics card driver can cause problems. Upgrade your graphics drivers to use hardware acceleration and WebGL

Update all your plugins (Flash, Java, etc.): http://www.mozilla.org/plugincheck/.

Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections.

Do the images have transparent parts?

It is a new feature in Firefox 11+ versions to display a single image centered with an almost black background.

It is added by this stylesheet:

• resource://gre/res/TopLevelImageDocument.css

You can look at this extension to set your preferred background color and remove the centering if you display a single image in a tab.

• Old Default Image Style: https://addons.mozilla.org/firefox/addon/old-default-image-style/