When relying on web-based E-Mail such as G-Mail, it can be frustrating that there is no 'new mail' signal. The Firefox x-notifier extension (formerly, Webmail Notifier) is one way to obtain that signal. WebMail Notifier is only one of several options for getting notified about new mail when using G-Mail's web-based mail.
There are security considerations when using such programs, particularly if you are on an untrusted wireless connection. You should do some research to make sure that using these does not hand over the keys to the kingdom.
Note: This page formerly linked to a Webmail Notifier script that sped up the process of checking for new G-Mails over dial-up modems. However, a Webmail Notifier update was not compatible with it. The script is no longer available.