The keyboard shortcut is much more convenient (and I'm used to it from opening a new browser tab).
Check for New Mail Quickly in Mozilla Thunderbird, Netscape or Mozilla
To make Mozilla Thunderbird, Netscape or Mozilla check for new messages fast:
- Press Shift-F5.
- In Netscape, Mozilla and Mozilla Thunderbird 0.9, press Ctrl-Shift-T.
Note that this will fetch new mail in all your accounts. To check only the current account, use F5 or Ctrl-T respectively.
(Updated March 2013)