Thunderbird missing top menus

[Leszek Lesner]

Am Mittwoch, 28. November 2012, 12:19:23 schrieb Lars Noodén:
> On 11/28/12, John Hupp <lubuntu at prpcompany.com> wrote:
> > press Alt-F to see if the File menu appears.
> 
> I hadn't thought to try that, it does show the menus.  Very strange.
> They all disappear again when anything is pressed.  This makes
> Thunderbird usable but very awkward in that the keyboard must be used
> not just the mouse alone.

Perhaps thunderbird ships with a unity global menu plugin that keeps 
activating itself.
Try to take a look in the addons manager and disable anything that stated to 
be a global menu integration or unity integration.