From the ABP Forum:Thunderbird 1.5 is no longer supported by Mozilla - with Firefox 1.5 support ended a while ago there is no longer a reason for Adblock Plus to support these old versions. The next major Adblock Plus release will support:
Firefox 2.0
Thunderbird 2.0
SeaMonkey 1.1
Songbird 0.2.5 (not sure about that, must check whether Songbird 0.2.5 really uses Gecko 1.8.1)
K-Meleon 1.1
Moblin Browser
For us this means that I can throw out some hacks (not quite as many as when dropping support for Gecko 1.7 but still). More importantly however, I can now use new JavaScript features - and some of them promise less code and better performance.
http://adblockplus.org/forum/viewtopic.php?t=1778