When using the compact_menu_2-3.1.1-fx_tb_sb_sm.xpi on a default Thunderbird 3.1 build for Linux, Tb throws an error:
Compact Menu 2 3.1.1 could not be installed because it is not compatible with Thunderbird 3.1
Latest version of CM2 is 3.2.0a
It's already compatible with Thunderbird 3.1
Milly, that version does not appear on the downloads page:
Please install from https://addons.mozilla.org/thunderbird/addon/4550/ "Beta channel" at middle of page.
Sorry I'm lazy. ;)