Hi LR,
I'm guessing that the prefs.jp, is a file (javascript, possibly) that held your details for filling in boxes. You don't say what your
odd little program
was that showed your details, but I guess prefs.jp is benign, and could be deleted quite safely if you have concerns about it.
I couldn't find any info on the FF addin you mentioned, so maybe the developer has withdrawn it, which is why it is not working. Was it a Beta? Did you upgrade FF?
As for your gMail and Yahoo snags; I try to stick to the mantra of: I do not need this, I do not need this... repeat as required
Sorry, not much help on that one.