Obtaining session cookies from a running Mozilla instance

I have various web applications which I want to automate with some
script, but they require complex login procedures which are rather
difficult to implement without a full web browser.

I have discovered a way to extract both long-term and session cookies
from the profile stored on disk (at least on GNU/Linux), and it works
really well with the specific Firefox versions I have tested.  But
this does not really look like stable API.  Is there a better way to
achieve my goal?

I want to avoid bundling a complete Firefox installation with my
automation scripts, I'd rather reuse an existing Firefox installation.