Doesn't it work if you toggle the online status via File > Work Offline?

You may need to toggle an extra off/on cycle if it doesn't work the first time.

You can try to create a new Boolean pref with the name network.manage-offline-status and set the value to false.
Right-click on the about:config page to open the right-click context menu and use "New > Boolean" to create a new Boolean pref.

Name: network.manage-offline-status
Value: false

I'm having the same problem. It's been happening the past few weeks (4/2012) and it's getting REALLY annoying having to go into the menu to turn off the 'work offline' mode almost every time I go back into firefox. And I stress the 'almost' because it's not at all consistent.

Just what IS this about:config page you keep telling us to change? Please tell us where to find it! I've tried applying the 'fixes' you folks keep telling us to apply in our terminal windows and none are even recognized, let alone successful. Could it be that we're using macs and you're not? Please, if you have a solution, please post a clearly-written set of instructions which walk us through the fix.