For anybody wondering how to do a simple mail merge in Mozilla Thunderbird there is a wonderful add-on available that makes it all very easy even for a dummy like me. I regularly send out news letters by email and have several hundred recipients on my mailing list, the Thunderbird mail merge add-on makes this as easy as riding a bicycle, unless you can't ride a bicycle, then it's as easy as something else that you can do easily. Well, you know what I mean. ;-)
You can download the add-on here:
Mail Merge 3.7.1
...and then simply follow the instructions lower down the page to install and use. Easy peasy!