This is an awesome add-on for Firefox. It's like having 2+ browsers within a single window. You can log into two different email accounts within the same window! You can open up two Amazon.com accounts via two different colored Containers! It's so awesome! User Interface Power to the MAX!

Personal Example: I have like 8 gmail accounts and it's a hassle to log into each account and check them via PC browsers (Thank goodness for the mobile gmail app, but sometimes you just gotta view an email from a PC). But with Firefox containers, I can easily open all 8 gmail accounts, each within it's own container. For example: my first, main gmail opens in a blue container. My second, secondary gmail opens in a red container. My third, junk email opens in a yellow container. I literally press the containers button, pick a color, open a new tap in said color, type in gmail.com or press the home button, and (since I have Firefox remember my passwords to my gmail accounts) it automatically loads just that email (for that color). In essence, I can check all 3 emails within a single browser window. Containers is convenient!

Technical Note: I've streamlined getting to my emails faster. I use another Firefox Quantum add-on, "New Tab Override" by Soren Hentzschel to actually get Firefox Quantum to automatically open a new tab specifically assigned to https://mail.google.com/. Now I can just: pick a container color (blue, red, yellow) and open a "new tab" in said color, and with "New Tab Override," I'm automatically redirected to https://mail.google.com/ . Yes, every time I open a new tab I'm redirected to https://mail.google.com/ but now I no longer have to type "gmail.com" into the url bar nor press the home button to load gmail for each of my container colors. That's 8 less clicks for me and I get to have Facebook be my homepage again!

Edit: Be careful when first opening up Firefox Quantum. You won't be in any Container. Instead you will be in a colorless container a.k.a. normal Firefox browser/window/tabs (vanilla Firefox). If anything, I ask that the developers make it so I can start Firefox in a specific colored Container. Also be careful when opening up a new tab in Firefox Quantum, the default new tab is a vanilla tab so you must hold the + symbol to open a specifically colored new tab.