My current understanding after using Bitwarden for a day is that the autolock after X minute interval starts counting from when the sidebar or password tab is open, as mentioned in Firefox auto-lock doesn't work
This seems to be rather counterintuitive, as the lock is intended to prevent other users from accessing the stored passwords if the computer is left unattended. If the user accidentally leaves the sidebar open and leaves the computer unlocked, an attacker with access to the unlocked computer will theoretically be able to compromise passwords, despite the auto lock feature being enabled.
Additionally, I can’t see anyone browsing with the Bitwarden tab open, it’s generally closed during browsing and only opened if a password check is required.
It would make more sense for the lock to trigger if no new pages are loaded in the last X minutes (or use the browser’s Idle API), instead of when the sidebar is closed. This is also the behaviour of Lastpass, which is more intuitive and IMO provides better security.