SeaMokey 2.17.1 Rated 5 out of 5 stars
I just updated to current version 0.51 from https://code.google.com/p/gt-tasks-sync/downloads/detail?name=google_tasks_sync-0.5.1-tb.xpi. With the modifications shown in http://code.google.com/p/gt-tasks-sync/issues/detail?id=30 Google Tasks Sync still works fine in SeaMonkey.
And reordering tasks is finally working, too.
Thank you very much!
My suggestion: Add the necessary code lines to make your extension fully compatible with SeaMonkey. All the users that do not intend to fiddling with xpi/rdf code file lines will be thankful.