Rated 5 out of 5 stars

The software itself is excellent, but an import / export feature would be a nice add-on!

This review is for a previous version of the add-on (2.9.6). 

It is. Literally: https://addons.mozilla.org/addon/password-exporter/

Several people are requesting this. But the question is, what's wrong with that other add-on? Besides, it's not only SPE users who want the functionality. Anyone who saves their passwords in Firefox and Thunderbird might want to back up their passwords. If they don't want to install SPE, they can still install the lighter-weight Password Exporter (PE) and get that functionality.

I can think of two things that can be improved upon in regards to PE: (1) working on SeaMonkey and (2) supporting another format, like JSON (nice to have, but not required). Other than that, what is there?

I won't definitely not do it. If I do, though, I might implement it as a separate add-on. So that would be a replacement for Password Exporter.