I just checked another site of ours that is using DNN 6.1 and it does the same thing when you hit the back button.. the menu disappears.

I managed to fix (workaround?) the issue. I had to change the menu control and menu provider I was using in my skin.

I replaced the menu control I was using my skin to MLMENU (a control that manages multi-languages). It's part of the PageLocalization module which you have to install (free download).

And I also replaced the current menu provider name "DNNMenuNavigationProvider" to "SolpartMenuNavigationProvider". Make sure you add a provider entry for this provider in your config file under "".

I had to also register the following 2 user controles in the skin too:

<%@ Register TagPrefix="dnn" TagName="MLMENU" Src="~/DesktopModules/PageLocalization/MLMenu.ascx" %>

<%@ Register TagPrefix="dnn" TagName="SOLPARTMENU" Src="~/Admin/Skins/SolPartMenu.ascx" %>

After all this is done, the menu shows up fine in all browsers and doesn't disappear in Firefox after hitting the browser's refresh or back button.

I hope this helps those who were as lost as I was.