This has been requested a few times (for intranet sites, for example), so I'll look into implementing it in the near future.

Started working on this.. I'll probably implement it as a 'hidden setting' (about:config) for now, as it should only be used by people that know what they're doing.

Fix implemented in 0.79. User can enable a unique alert for self-signed certs.

https://code.google.com/p/https-finder/downloads/detail?name=httpsfinder079dev3.xpi

Issue 13 has been merged into this issue.