We currently use the caching engine from dnssec-validator.cz but it is not sufficient for our purposes (caches per window only slow things down too much with synchronous lookups).
I'm not aware of a nice way to implement this in Firefox, sticking with per window caching.
Also, asynchronous lookups are rather nice for responsiveness. (addon runs in same thread as gui)
We currently use the caching engine from dnssec-validator.cz but it is not sufficient for our purposes (caches per window only slow things down too much with synchronous lookups).