I use Komodo Edit.
I also use SQLite Manager, but as a Firefox extension, not standalone. It does not need to be an FF extension, but was easier to install that way.
All the XUL apps I have installed (FF, Thunderbird and Komodo Edit) seem to have their own copies of all the XUL stuff (everything that used to be in XUL runner).
In the meantime XUL runner seems to be deprecated in favour of using Firefox with a -app option, and what were formerly fairly popular XUL apps (Prism, Songbird) seem to be dead.
Has everyone other than Mozilla and Activestate (Komodo Edit) decided that there are better ways of building cross platform GUI software?