The problem was first mentioned in this thread, and I have personally verified this with the English version of Firefox Portable 11.0.1 and the latest development build of Adblock Plus: forum/viewtopic.php?f=1&t=9967
The submenu is available from the icon in the Status Bar but not from the Tools menu, as it is in Firefox; if this is a known issue, or a design decision that won't be fixed, I have not seen mention of this before.
[Fixed] Submenu fails to load from Tools on Thunderbird
[Fixed] Submenu fails to load from Tools on Thunderbird
There's a buzzin' in my brain I really can't explain; I think about it before they make me go to bed.