Adobe stopped supporting Flash for Linux (even in Firefox) around version 11. They no longer produce any 'new' Flash releases for Linux and the way Firefox handles plugins. Due to the really really old version number (18 is the latest Flash overall, and 11 is the last supported Firefox plugin version available for Linux), and other security concerns, Firefox automatically disables these 'old' versions. This applies to all Operating Systems, not just Ubuntu and *nix. (While @ParanoidPanda is correct they now enforce that for a few extra versions across all platforms, this isn't the primary reason for this warning in Ubuntu/Firefox).