If you like to have your entire computing experience within your Firefox window, you might be interested in accessing your POP email accounts directly in Firefox. The Simple Mail extension gives you this functionality, and true to the name it's very simple.
After installation, you should notice the little mail icon in the status bar, which will highlight and display an unread count if there are new messages, but first you'll want to right-click it and open the preferences to add your new account.
Here you can add a new account or import from Thunderbird (a feature I did not test)
Left-clicking on the mail icon will open the mail client interface, nothing more than a two-paned display of the messages sitting on your server. You can choose to delete the individual message through the right-click menu.
The new or reply options bring up the new email dialog, which is also really simple.
I would personally only use this as a pop mail notifier, as the client interface is just far too simple for any serious usage. There are much better options that you could use:
- Forward your POP email directly to Gmail and access it from there.
- Sign up for Google Apps for your Domain and use Gmail with your own email domain.
- Use Gmail's Settings\Accounts tab to add in a POP email account that gmail will check for you.
You can use any of those options in conjunction with the excellent Gmail Manager extension for Firefox.