Rated 5 out of 5 stars

This add-on provides exactly what I was looking for - a context menu option for choosing which browser to open links with.
I am now using it with both Firefox and Thunderbird, on 64-bit Windows 7.
Tip 1: The add-on Homepage explains how to create a "Launchy.xml" file for manually configuring the context menu items. A tab in the Add-on Options shows the path of the chrome folder into which the launchy.xml file should be placed. If there is no chrome folder in that location, create the folder.
Tip 2: If the Launchy context menu indicates that no applications were found, it may be due to a syntax problem in the launchy.xml file. Don't include double quotes around the program path. Backslashes and spaces within the program path seem to be ok. After updating the launchy.xml file, close Thunderbird and restart it for the changes to get picked up.