Talking about vertical tabs, I was in the middle of studying for an upcoming exam, then when I alt-tabbed back into Firefox, all of my tabs are missing with that unsupported addon error. Fortunately refreshing Firefox gave me back normal tabs, at a cost of uninstalling all of my addons.

The problem is that Tree Style Tabs relies on userchrome.css edit to hide the tab bar, and when TST is forcibly removed there is no way to access the tabs, because that edited userchrome.css is still there. This is very disruptive. At least with the pre WebExtension addon TST itself hides the tab bar, so if TST is removed then the original tab bar comes back on automatically