The adventure continues.
I create links using CKEditor. I view them in source (screenshot attached). I send the email. The recipient clinks the links. However, instead of going to the URLs specified, their email contains links back to the sending site (me). When I view the email in FireMail after sending, sure enough, the URLs were rewritten en route.
Hmm.
EDIT: Checked the Firemail database tables. The links as I originally entered (wanted) them were saved correctly.