on Firefox restart, tabs in specific group(s) moved to "Ungrouped tabs"
When using Tree Tabs with latest public version of Firefox Quantum, on restart of Firefox, Tree Tabs will sometimes (often, probably 1 out of 3 or 4 restarts) move tabs that have been placed in a specific tab group into a generic tab group called "Ungrouped tabs."
For example, there were approximately 50 tabs in a tab group called "Research." After restarting Firefox, there are 0 tabs in the "Research" group and those tabs are (mostly, although I haven't verified each one) now in the "Ungrouped tabs" group.