Uninstall any other versions of Thunderbird you may have on your computer. Click on Finder (Face icon located on Dock-bottom-left-hand). Highlight "Applications". Find the "Thunderbird" icon and drag it down to your "Trash" icon (garbage pail located on Dock-bottom-right-hand). Also, on the left-hand pane, click on the username you used to sign into the computer. Double click on the Library folder and select "Thunderbird folder entry" and drag it to the "Trash" icon.
You will see a "You have chosen to open Thunderbird 3.0.3 dmg"’ window pop up. Select "Save File" and click on OK.
Install and Configuration
Go to your “Downloads” and double-click on Thunderbird 3.0.3.dmg file.
A "Thunderbird" pop-up window will appear. Click on the Thunderbird Mail icon and drag, following the arrow, over to the 'A'pplication file folder icon. The system will copy Thunderbird to the Applications folder.
Double click on the Thunderbird Mail icon. A newly created, bouncing, Thunderbird Mail icon will appear on your Dock.
A pop-up window will appear. Select "Skip this and use my existing email".
Type your name, your email address and email password in the provided fields. Optional: place a checkmark in the "Remember Password" box. Click Continue.
Thunderbird will attempt to find the account. Wait until you see (do not click) the Re-Test button appear at the lower right.
Setup the screen as seen above. Do not modify your personal settings (name, username, password). Click on Re-test button to verify the settings. Click on the Advanced config button.
On the left hand side, under your email address, click on Server Settings. Click on the Advanced button at the lower right.
Verify all the settings are the same as the above screen.
Click on OK twice to exit all setup windows.
Exit Thunderbird. Thunderbird is now configured to access your Mymail account.