Profiles are used by Firefox to store data that is specific to an individual user.
If you think you might want to share a profile between a Windows computer and a Linux computer, you can go to http://sillydog.org/netscape/kb/linuxwindows.html, which will give you the details. If you are running both platforms, I recommend you visit this site.
Web security is important. You must protect yourself, your family members, and your property from attacks. Firefox has some innovative security features, such as allowing you to configure various security settings.
Some sites are written for Internet Explorer exclusively, but virtually all sites display acceptably in Firefox.
Pop-ups, pop-unders, and pop-overs are all annoying ways to try to sneak advertising onto your computer.
Adblock enables you to import, export, and remove filters for advertising content suppliers.
Try to get your extensions from the official Mozilla website (Mozilla.org), but some interesting extensions are located at other sites, too.
You can choose to allow or prohibit software installations based on URLs using the Web Features section of Firefox's Options dialog box.
If registering and providing personal information bugs you, use BugMeNot an extension for Firefox that allows you to register anonymously.
Cookies are often misunderstood and maligned. Actually, cookies are not the Darth Vader of the Web!
Firefox's master password lets it encrypt stored passwords and allows users to have to remember only one (master) password.
"Determining the Real Location of Sites" is an excellent discussion of how websites and HTML email mask the true destination of a hyperlink. You can find it at http://www.michaelhorowitz.com/linksthatlie.html, a very interesting site.