It usually happens when the site is experiencing issues which prevent it from loading properly. My personal website sometimes looks like that when the host is under a denial of service (DoS) attack. Clearing Firefox's cache may solve the problem.
Clear the cookies and cache
Firefox caches websites, which means it saves some files on your machine so that it doesn't have to re-download everything when you visit sites. If a website has updated their code, Firefox may still be using some of the old code along with some of the new, which can make a web page display incorrectly. To fix this, you need to clear the cookies and cache:
- Click the Library button , click History and select Clear Recent History….
- In the Time Range to clear: drop-down, select Everything.
- Click the arrow next to Details to display the list of items that can be cleared.
- Select both Cookies and Cache.
- Click Clear Now.