Nowadays, many people maintain separate email accounts for work, for family and friends, to post into forums, and so on. I use several Gmail accounts, and I find it a challenge to read all those accounts without having to set up a POP client or disconnecting and connecting to the Gmail Web site with a browser over and over again. Gmail Manager is an OS-independent Firefox extension that can help anyone with that problem. With it, you can manage multiple accounts and be notified as soon as you receive a new message in any of them.
To install the extension, go to either the official Gmail Manager Web site or to the Mozilla add-ons site. Click on the install button, follow instructions, and restart your browser. If everything ran fine, you will have a new "M" icon at the right of the status bar.