There are some odd display issues with OSX, but I've never heard of this one. If you open Tools/Account Settings and select the account in the left pane, do you not see the Signature text box under your name and address?

Try running in safe mode (Help/Restart with add-ons disabled) in case you have a broken layout or hardware acceleration is the cause. What is the resolution of your display?

Thanks for the reply. Yeah, no box whatsoever [see attached screen capture]. The REST of the options are there, except the one I need.

Resolution is 1440 x 900. MacBook Pro 2009. OSX 10.6.8

Besides the signature box, your settings window is missing the selection drop-down for the outgoing server. If safe mode doesn't make any difference, I suggest you reinstall the program. The last version to support OSX 10.6.9 is TB 45:

http://ftp.mozilla.org/pub/thunderbird/releases/45.8.0/mac/

I don't think your resolution is causing the problem

http://kb.mozillazine.org/Toolbar_or_windows_layout_not_working

Thanks! I've been looking around but all I can find is the latest release. Any idea where I can download older versions Thunderbird?

Update — sorry! Never mind! lol