You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you but it doesn't work either. Been re-installing my PC OS too yesterday. I'm done your tips but still the facebook page didn't load normally.

Have you noticed any issues with secure sites, such as Firefox rejecting their certificates? I ask because you can end up with unstyled pages like that if you add an exception for a site, but the site uses different servers for style sheets and other important page components.

If that sounds like it could be what happened, let's try to get to the bottom of the certificate issue. If you click the padlock (or gray exclamation warning triangle) in the address bar for Facebook, then More Information, then View Certificate, what do you see under "Issued by"? I have attached a screen shot of what I see for comparison.

I alreadey checked it and compare it with your screen shot and they are same.

With Facebook open, could you open the Web Console in the bottom of the tab by pressing Ctrl+Shift+k. Along the top of the console you should find a button which says Network. Click that, then reload the page and see whether Firefox loads the CSS files that style the page.

So I've checked it and instead of loads css files they all shows "PLAIN" and 0 KB in size.

So it's already load in normal way again. Thank you for your help. Without your help I can't locate the problem. Actually the facebook suddenly load normally, after I do something, but if in case that problem occur again in the future can you tell me some step to deal with it? Thank you very much.

I'm not sure what went wrong or how it got fixed there. A "text/plain" type could indicate there was a problem with a cache along the way. If it happens again we could explore further.

Well it happens again.

I'm opening Facebook right now. At first attemp Firefox doesn't load the CSS files again. But a few second later, after opened this page I'm going back to Facebook in the other tab, and reload the page. Then Firefox loads the CSS files and Facebook appearance was normal.

Firefox normally will try to cache those files (in my screenshot the "304" status indicates Firefox checked whether the cached files were up-to-date and FB reported they were), so one possibility is a problem with the cache. Do you have Firefox cache files normally, or do you use any of these:

• Private browsing when visiting Facebook, so cached files are discarded between visits
• Clear cache when Firefox shuts down (Options > Privacy)
• External software that cleans out browser data

Another possible reason would be a connection issue, perhaps using a proxy server or private VPN, or some kind of accelerator that caches files separate from Firefox's cache.