In an effort to better integrate into Thunderbird and also to clean up some very evil workarounds we have reworked the Menu Structure of Lightning. Work on this was completed yesterday in bug 456385.
Previously, we switched the contents of the whole menu bar when changing from mail mode to calendar or task mode. Aside from the fact that this forced us to use very heavy workarounds for certain platform-specific bugs, this only made sense for 0.9, since the modes were very dedicated. I believe this was also a nightmare with regard to accessibility.
To fit better with the new tabbing mechanism available in the Thunderbird 3 betas, we changed this mode switching logic to show each mode in a tab. It looked a bit awkward that the menus change, just because you switched to a different tab.
Therefore we aimed at an integrated solution that has a fixed menu for manipulating events and tasks, and sorted our other menu items into the existing menus. Aside from that, the idea is to provide buttons and other actionable controls right in context – the user shouldn’t have to travel far to do what he wants to do.