The toodledo add-on is only intended for FireFox, not Thunderbird. And
you need to go on the toodledo site (and not on addons.mozilla.org) to
find a FF 3.0 compatible version, so I guess this add-on is not a very
high priority for toodledo.
By the way, don't expect some magical synchronization add-on for
toodledo and Lightning's tasks. Their model is indeed similar (after
all, they both manage tasks), but there are different enough that
advanced cases would not work (example : the recurring tasks).
Pascal (a toodledo and Thunderbird user)