Quick reply... Fennec 18 on Fremantle, for some reason, depends on thumb0 versions of libqt libraries. Ideally, there should be separate builds of Fennec 18 for non-CSSU Fremantle, for CSSU-stable, for CSSU-testing, for CSSU-devel and for CSSU-thumb. I have CSSU-testing installed. Best wishes.