you can toggle the sidebar! you can also customize it through the sideberry extension (i also managed to make it autohide once but i don’t really remember how!)

the context menus thing may be because you are using the “default” firefox theme thingy! it also just looks greater with firefox color for some reason…

EDIT: i found how to! go to about:config and set a new boolean entry for uc.tweak.hide-tabs-bar, that should make it autohide!