created a new profile, installed Lightning, moved to a mail folder (it shows the "Welcome to Shredder" page) and Ctrl + +/- now works without opening first the Zoom menu.
Created and saved a mail, closed and restarted TB with the message tab open, Ctrl + +/- works.
Now seems working on Thunderbird 3.3a4pre (Miramar):
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0pre) Gecko/20110405 Thunderbird/ 3.3a4pre
Lightning 1.0b4pre 20110418
created a new profile, installed Lightning, moved to a mail folder (it shows the "Welcome to Shredder" page) and Ctrl + +/- now works without opening first the Zoom menu.
Created and saved a mail, closed and restarted TB with the message tab open, Ctrl + +/- works.
Could someone confirm?