If you run firefox in ubuntu, maybe all serials, running firefox by firefox -ProfileManager, check work offline and press enter, then in the firefox menu the " work offine " is still unchecked.
Which version of Ubuntu/Firefox, as I cannot reproduce it 2 of my machines using 14.04 fully updated.
So it's congratulations they fix this long term bug in new version Ubuntu, I'm using 12.04, may update every some weeks