You can run multiple browser profiles, or use something like Firefox Containers to sidestep this pain point. I use my own profile on a hot seat shop PC, and every user has their own desktop shortcut, which directly opens the appropriate profile in the browser. I can be signed in to multiple different Google accounts simultaneously in this way. Signing out signs out for every Google account in that browser profile; adjacent browser profiles' sign-in state is not affected by signing out on first profile. Hope this helps.