The website listed below brings up a panel that says "This page requires AC_RunActiveContent.js." and won't run.
Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
"Remove Cookies" from sites causing problems:
Start Firefox in Troubleshoot Firefox issues using Safe Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).
Clearing the cache fixed it. Didn't have to mess with the cookies. Thank you very much. I couldn't imagine that the cache would cause such behavior.
You may have had a corrupted file AC_RunActiveContent.js in the cache that was working.
First to try is always to reload and bypass the cache in case of unexplained problems.
Reload web page(s) and bypass the cache.