Firefox Again

Well, Firefox usually does a better job of displaying sites than Chrome (though Chrome is without a doubt faster). If you are having problems with all or most sites using Firefox, the problem is on your end. I would try uninstalling Firefox (I know you already did this), then, go download a free program called "Glary Utilities" (I'm assuming that you are running some flavor of Windows here), run that program, have it clean your registry (this program is safe). Then, after all registry entries for Firefox have been cleaned, try reinstalling it.

BTW, why not use Chrome? I admit it does have a few issues from time to time, but in terms of speed, there is nothing out there close.

If Firefox is messing up all the pages, is it possible it's ignoring something because a setting got toggled somehow?  Is there a FireFox accessibility setting, for instance, that tells it to ignore style sheets and replace them with some default style?

I guess if it's working (weird that it is, so suddenly) I'd stick with FireFox.  You always have the option of Opera later.  I'm not really up to speed on either, as I dumped FireFox about a year and a half ago for Chrome.