I should clarify that I prefer using desktop applications and would rather not have to chat in a browser window (even if the desktop application is technically a web browser internally).
Ferdi [0] is FOSS and supports multiple accounts for the same service [1].
[0] https://getferdi.com/ [1] https://help.getferdi.com/services/multiple-accounts
[0]: https://addons.mozilla.org/fr/firefox/addon/multi-account-co...