"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now" "Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies" If this doesn't help, try ending any Firefox processes and completely restart it. If none of this helps you might consider you could have Malware or something nastier. Connection issues can also cause that error message. I remember having the same issue a couple of years ago and had to completely restart my PC for some reason to get the new Firefox to install. Sorry I don't have any more in depth answer. Hope this helped you, have a nice day.