Yesterday I made Forumzilla index items in its datastore by ID rather than URL. I did it to fix bug 12132, which causes Forumzilla to ignore multiple items with the same URL in feeds like the wiki.mozilla.org Recent Changes feed.
This is a pretty significant change to be taking on the stable 0.5 branch, but I'm doing so because I think it's the necessary minimal fix for important feeds. Since not all feeds provide unique IDs for their items, I generate them if necessary by taking the SHA-1 digest of each item's URL, date, title, and description.
To prevent Forumzilla from redownloading already downloaded items, I wrote some code that checks to see if new items are actually old items that were previously indexed by URL. If so, I just convert the old record to a new, ID-indexed one and don't redownload the item.
With the changes, the aforementioned wiki feed now works, and my regular subscriptions all seem to work, too, but the changes will need more testing to make sure I haven't regressed any feeds in the process, so I built an 0.5 branch development package you can use to test the changes. I've tested it in Thunderbird 1.0, Thunderbird 1.5 (release candidates), and Mozilla 1.7 on Linux. Give it a try in your own favorite compatible mail client, and let me know how it works for you.