Review for Mail Merge by Steve Hards
Rated 5 out of 5 stars
Have only used the Mass Mail feature and it worked beautifully. I created an email as a template, called up one and pasted in the list of recipients. Clicking on File > Mass Mail generated each individual email and put it in the Unsent emails folder, ready to edit or send.
NOTE: If using this add-on, it may be best to mail only 50 or so people at a time and have an hour's break between batches so that your ISP does not ban you for spamming.
To create your own collections, you must have a Mozilla Add-ons account.