For love nor money, I cannot switch to a separate profile in the browser and have the M365 portal sign in with the account associated with that profile. It will REPEATEDLY default to the one signed-in to Windows itself and even if I logout, close the window, open a new one (under the correct, different profile) and login with the correct one, when I try to launch the Exchange Admin Centre (for example) I am presented with a page from my employer's tenant - not the customer one.