Working on the assumption you have made no other browser/firewall/anti-virus changes, try following these steps.
Log out of the forum and browse to a different site. It's not critical which one, just be sure to log out and then leave this particular site.
In the Firefox Menu, click on Tools
In the dropdown menu, click on Clear Recent History
In the resulting dialog box, make the following selections:
Time range to clear: Everything
Be sure the following checkboxes have ticks in them:
Browsing and Download History
Form & Search History
Cookies
Cache
Active Logins
Site Preferences
Click on Clear Now
Once everything has been cleared, just pay close attention, it shouldn't take more than a few seconds, shut down Firefox. Close it clear down.
Once Firefox has finished closing, re-open it, return to this site, log back into your account and check the thumbnails.
It is a good idea to clear a browser's cache on a regular basis. I'm online all day, every day, so I clear cache at least once a day. You might consider doing it every other day or at least once a week. This holds true for all browsers, not just Firefox. A browser can often download a page from a site and then continue using its cached version of that page, every time you visit it. Changes can be made to the page and a browser can stubbornly continue using the older, cached version.