How to fix "Firefox is currently in offline mode"
The simplest solution how to fix error message about Firefox offline mode is going to the File menu a unchecking the Work offline option.
However, this shouldn't help every time, moreover the issue with Firefox' offline mode could come back and persists. The problem may be in your proxy settings. So if the File > Work offline unchecking doesn't help, go to the Tools menu and click the Option item (on Mac OS go to the Firefox > Options menu).
Switch to the Advanced tab and click the Network button. Continue with Connection settings button and set up the No proxy option. Now save the settings, restart your Firefox and hope the problem with offline mode has been gone.