[mbroshi]

My favorite hidden feature in Firefox is having multiple profiles[0],[1]. Unlike containers, profiles have separate bookmarks, extensions, etc. I have found profiles even more useful than containers for development.

[0] about:profiles (paste into URL bar in Firefox)

[1] https://support.mozilla.org/en-US/kb/profile-manager-create-...

[javajosh]

This is a feature of Chrome as well, although it isn't very hidden. https://support.google.com/chrome/answer/2364824