Comment 1 for bug 148696

I can confirm this problem. The package installation may fail for example because Universe repo is not enabled (see bug 147744) or because another package manager is still running; and it seems that in all those cases first a correct error message will appear but then another dialog will say that plugin installation has finished and Firefox has to be restarted. Which is clearly wrong as the plugin was in fact not installed and restarting Firefox will not make it available.

I suppose this bug could be marked as Low priority (because the user already got a hint that the installation failed) but it should at least be set to Confirmed.