Firefox Profiles allow you to have multiple instances of Firefox on a single computer. Each instance can have their own extensions, settings, themes, cookies, browsing history, etc.
By default, there's only one profile on Firefox. To create a new one, open Firefox and type about:profiles in the search bar and press enter.
Click on "Create a New Profile" and follow the steps. Once it's created, you can launch it by clicking "Launch Profile in new browser".
If you use multiple accounts for a website, it is simpler to use Firefox's Mutli-Account Containers.