Are you missing images and does Facebook look like a plain website with no CSS stylings?

Try to delete your cache and cookies.

Cache

• Press the CTRL key, ALT key, and DEL key to open up the Clear History window
• Unselect everything except Cache and set the time range for Everything
• Clear Clear Now

Cookies

• Go to the orange Firefox button > Options > Privacy > Remove individual cookies > Find and select Facebook.com and click Remove Cookies.
• Restart Firefox and reload Facebook.

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.