Now, this is one of the best features of Mozilla Firefox. Almost all of my time online I need to have many browsers open at the same time, and honestly, it can get quite messy. Mozilla Firefox has however added a very helpful function to the browser, called tabbing.
Open your Mozilla Firefox browser, then press "Ctrl-T" at the same time. Your browser will now open a new tab that can be used, in the same browser window.
As you see by pressing "Ctrl-T" I now have 2 tabs in the same browser. :) You can alternate between these by pressing "Ctrl-Tab". If you need more browsing tabs simply press "Ctrl-T" again.
If you want the browser to open multiple sites when you start the browser, this is possible. Go Tools > Options...and you will see this choice in the "General" selection:
As you see the browser here gives you a variety of options, you can 1. use the pages you're currently browsing; 2. Choose startup pages from you bookmarks; 3. use a blank page. As you see in the screenshot, you can also set the startpages manually, and if there's more than one, divided them with a | mark, and Firefox will open them in separate tabs. Use as many pages as you like. :)
There are more ways to control how tabs work as well, if you right-click on a tab, you'll see something like this:
Here you're able to create new tabs, reload the current tab (reload is also available via "Ctrl-R"), reload all tabs, close other tabs and finally close tab (also available via "Ctrl-W").