Normally they'd use different user accounts on the computer. If that isn't an option, you can set up separate Firefox profiles:
https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
Those are conceptionally the same for Zotero, though you need different filepahts/names, obviously for running it.
A straightforward approach would be to use Zotero for Firefox for one user and Zotero Standalone for the other (and be sure to indicate that they should not share databases when prompted).
There's really no reason not to just use separate user accounts. Every major OS supports it, and it keeps all data separate, Zotero or otherwise. There's no need to do something special for Zotero.
https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
Those are conceptionally the same for Zotero, though you need different filepahts/names, obviously for running it.