When I installed LibreWolf via the Microsoft App Store (to easy auto updates) it would not let me into a profile file. It would state a directory in the dialog. I would try to then manually go to the directory and it would not exist. I don’t know if that was a result of a sandbox AppStore design or what. Anyway, I uninstalled and am trying Zen out. So far, so good and it has vertical tabs like I used Sidebery for in FF.