How I prevent Firefox from doing this all the time?

Looks like your page style is getting set to no style. Go to View -> Page Style and click on No Style and tell us if that's what you're experiencing. If so, I have no clue what could be causing it. What plug-ins do you have installed?
 
NoScript or some other script blocking tool can easily prevent the CSS file from doing it's job. I don't note a NoScript icon on the status bar in the pics above, but there could be something blocking content from those websites. That would be my first guess as I know that when I typically visit "new" sites I've never been to before, with NoScript blocking the page scripting, they'll typically have that kind of lackluster look.

Check your addons/extensions for something that could be blocking content from some sites.
 
Looks like your page style is getting set to no style. Go to View -> Page Style and click on No Style and tell us if that's what you're experiencing. If so, I have no clue what could be causing it. What plug-ins do you have installed?

Linkification and Greasemonkey and that's it. If I click NoStyle this page becomes like a document lol.

Even when I disable both, it's still like that.
 
Are you having this problem on a computer at work? I've seen something like this before that was caused by the way our IT department set everything up.
 
Are you having this problem on a computer at work? I've seen something like this before that was caused by the way our IT department set everything up.

Nope.

Can it be that my computer just sucks ass and I need a new one?
 
It's not your computer. You said yourself the pages work fine in IE. Try uninstalling Firefox, deleting the profile folder, and reinstalling.
 
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