If all sites are displaying this 'problem', it's a function of the preferences in FF. Might be worth moving them to the desktop and starting FF up again. Before doing that, I'd check for CSS.
If it is only one specific site, it is a function of Cascading Style Sheets which is the more modern and capable way of specifying all the stying functions of a web page. You can see if the page uses a style sheet by looking at the information in the header of the source for any page. If you'll kindly give us a link to the page, we can do that quickly.