Using Browsers other than Tor Browser in Whonix.
In Whonix, for better anonymity it is recommended to use only Tor Browser for browsing the internet. Use of any browsers such as Chromium, Firefox , Opera and others is discouraged. Reasons for that are elaborated on the Tor Browser wiki page.
Whonix is The Everything Tor operating system (OS). All internet traffic is routed through the Tor anonymity network, without exceptions. Whonix is the "All Tor Operating System", featuring reliable IP hiding.
The reason why browsers other than Tor Browser are discouraged is because hiding your identity is harder than just hiding your IP. This is elaborated on the Tor Browser wiki page.