It can be helpful to have custom themes for your browser(s). Especially when you are doing a migration project. You need to be signed in to multiple browsers and make sure you keep track which one is signed in to which tenant.
You can even create your own themes pretty easily.
Go to https://addons.mozilla.org/en-US/firefox/addon/firefox-color/ and add the add-on.
You will be able to customise your theme from https://color.firefox.com/.
It is that simple!
