perhaps you have Thunderbird in offline mode. IS there an image a little like ((o)) on the bottom left of your Thunderbird main screen if it has a red mark on it you are offline. Clicking the image changes the state.

Toggled online on/off symbol. "Send" button still grayed out. The TO: address appears in a red block with white letters. The address book has valid Email addresses which I can put in the TO: box

You mention just installed Thunderbird.

Please check the Firewall on your computer. Access Firewall - this will be either the default that came with computer or it might be controlled by the Anti-Virus program you install.

Thunderbird needs to be set up as an allowed program otherwise it might get blocked. So make sure this is done. As an example: I'm using Norton 360, I've included an image below which shows how I've set up the Firewall to allow both the thunderbird exec file and the updater exec file.

Typically, the "Send" button will be grayed out if you are in 'offline' mode as described by Matt. You should see ((o)) if online.

But it will also be greyed out until a valid email address has been entered in a TO field and a pill created (press enter)

You mention : The TO: address appears in a red block with white letters. If you are using Addons & Themes > Themes: System Theme Then the red background may be part of something you selected on your computer. In the main view above the Folder Pane, locate the button called 'New Message' Does that button also have a red background with white font ? If yes, do not worry about it being red, it's a setting you have set up on computer. But we need to know to be certain, so please report back.

Have you installed any addon extensions ? If yes: What addons have you installed ?

Please test the following: Help > Troubleshoot Mode Clickon Restart and then select 'Continue in Troubleshoot mode'

If Send is now enabled when you have entered a valid email address in the TO field then you need to see which addon is likely to causing the issue. You may need to reinstall addon, but it may be a problem you need to report to the author of the addon extension.