Power internet users often will have several tabs open in their browser at any given time, and for someone like me who is always online, I'm constantly switching back and forth between tabs.
Firefox offers a few nifty ways in which you can organize your tabs to make your browser landscape a whole lot cleaner.
You no doubt have a few websites that you use constantly and are always open in your web browser, like your e-mail inbox, Facebook, or your calendar. App tabs simply reduce the amount of space the tab takes up on the tab bar, showing only it's favicon (the little website icon).
To make one of your tabs an app tab, right click on the tab and click on "Pin as App Tab". To undo this action, click on "Unpin as App Tab" and it will return to normal size.
Tab groups do just that: they group your tabs. This is really useful if you're working on a specific topic and don't want all your tabs mixed together. On my browser, I have tab groups for all my clients and whatever else I may be working on, and it allows me to keep several web pages open without everything getting super cluttered.
There are two drawbacks to using tab groups, however: