If you want the exact equivalent, use Firefox profiles (-p at startup, or install the profile switcher addon: https://addons.mozilla.org/en-US/firefox/addon/profile-switc...).

But Firefox has a much better feature: tab containers. You can attach a label to a tab, and only tabs sharing this label will share states, such as cookies, cache, localstorage and so on. This means you don't need to open a new window. You can have 2 HN accounts with each a different user logged in, in 2 tabs, next to each other, distinguishable with a color.

You can use it by activating a few settings in about:config (https://support.mozilla.org/en-US/kb/containers#w_for-advanc...), but the easiest way it to use the addon : https://addons.mozilla.org/fr/firefox/addon/multi-account-co...