The websites are very basic, with lots of white space.
Compared to other browsers or in general?
You could try to start firefoix with a new profile to check if a tweak/setting is the cause. Run firefox.exe from commandline as follows
firefox -P --no-remote
There create a new profile and start this.
Don't forget to untick "Don't ask on next startup..." to be able to swtich back to your current FF profile.
Since so many people seem to have problems with the newest "Stable" Firefox Builds... perhaps you should try out the "experimental alpha build" Aurora
Just make before a backup of your profile folder.
Seems in that Version 20 the devs have made a much more stable and faster browser as V18 at the time is able to deliver.
Very recommended and worth a try
Can you attach a screenshot?
Use a compressed image type like PNG or JPG to save the screenshot.
Make sure that you haven't enabled a High Contrast theme in the Accessibility settings and that you allow pages to choose their colors in Firefox.