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See https://addons.mozilla.org/en-US/thunderbird/addon/message-pane-button/ for a more recent add-on offering comparable functionality.
I missed the Toggle Preview Pane terribly. I'd didn't know how to "up the maxVersion in the rdf to 6.0.0" as per mod_wastrel's glimmer-of-hope-for-me post. Here's a link to instructions: http://www.techsupportalert.com/content/how-update-firefox-thunderbird-addons-new-release.htm
Toggle Preview Pane is installed now and working just fine.
I just upped the maxVersion in the rdf to 6.0.0**. (I also created a new 18x18 button image using the new Fx/Tb "standard"--looks better... one size fits all. :D)
**The simpler way nowadays is to just install Add-on Compatibility Reporter (which, primarily, disables checking for compatibility).
Small and indispensable