Comment 36 for bug 805136

I run into the same problem on a Samsung NC10 with Ubuntu 10.04 and Thunderbird 3.1.15.
A bypass for linux users is to use the 'xrandr' command from the command line to change the virtual resolution of the screen.
For example, the command:
xrandr --output LVDS1 --scale 1.25x1.25
on my machine modifies the apparent resolution from 1024x600 to 1280x750, which is enough to see and use the hidden parts of the menu.
To revert to the original, simply write:
xrandr --output LVDS1 --scale 1.25x1.25
HTH
Patrr