This is one of the best available features of Firefox. As I'm a Web developer, it makes my life much easier by allowing me to sign into multiple accounts with the same browser, all without increasing resource consumption beyond the level of a normal tab.

And as a user of online services, once all of my different user accounts are untangled, I know I'm less likely to be tracked by all of the services I use when I'm elsewhere on the internet.

Check it out! Firefox is a great browser, but I'd say this one feature on its own justifies trying Firefox, certainly so for anyone who tries to be conscious about their privacy on the web.