I'm thinking something similar to nsILoginInfo, where additional fields can be added. The current API design is very limited to a single consumer. Weave (for example) would like to sync form history. Right now that's done by reading from the database, but that's not a great solution. Also, Weave doesn't sync the data that makes the data even more valuable (lastUsed, timesUsed), because we don't want to make direct insert/update calls. So if AddEntry could take an nsIFormHistoryInfo, then we could just call that and it would combine as necessary (or update). Methods for getting data would also return arrays of (or just) nsIFormHistoryInfo. This would also make it easier for other consumers (e.g. nsFormAutoComplete) to get data because they wouldn't have to access the database directly.

I think a container + APIs like logim manager uses (well, the newer property-bag flavors) makes sense, and should let us cut'n'paste some of the existing code. Might be interesting to look at not implementing this API in XPCOM... Just a jsm with similar APIs, JS objects instead of nsIPropertyBags. Could do a shim layer (exposing XPCOM apis, using the .jsm internally) if the existing interactions with native code are tricky.