There is a way to hide email folders! At least for IMAP accounts.Right click on a root email account name in the email accounts list on the left side. Select "Subscribe" from the context list. Uncheck the folders you don't want to see. Select OK.This...
I'm assuming that this setting never was comparable to the 'always use private browsing mode' on the desktop — in my use it has been common to accidentally open standard tabs on Android and there never has been any persistent private mode in Fenix.
Mozilla likely will be implementing a native shortcut for the similar 'duplicate tab' feature, albeit that function can be executed rather trivially using a keyboard (focus on the address bar, Alt + Enter).
