In many services, we always find a form which should be filled in, including Internet. You will have to fill your username/password to check your inbox or to access certain information. In many cases, password is being made as case sensitive, meaning that "hello" and "HELLO" is different. The problem is that sometimes, we push the Caps Lock key without ever knowing it, so the password is typed as capital letters, where it should be in small letter. Thus, the system refuses to let you in, since you don't provide the correct information to the system.
Well, Daniel Glazman has been kind enough to provide a Firefox extension which will notify you when you have an active CAPS LOCK key, called Capswarning. This extension will add a new visible label (with red text) in every dialog prompt which ask for password if CAPS LOCK key is active.
Here's a screenshot, taken from the original website of Daniel Glazman: