Firefox displays that message when there is a combination of circumstances. Usually the server instructed Firefox not to cache the page and always to request a fresh copy AND usually the page was generated based on information you submitted using a form. If it was an ordinary request instead of a form submission, Firefox normally just gets the page again without bothering to tell you the page expired.

Is there a particular site where you have this problem?

Hello, As far as I can tell it is a random happening with no particular page or web site.

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

• Switch to the DEFAULT theme: Firefox menu button/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

You can check the connection settings.

• Tools > Options > Advanced > Network : Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can