User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.108 Safari/537.36

Steps to reproduce:

I have this RSS: https://getlibriarss.azurewebsites.net/api/RssTorrent
Import it and compare the Thunderbird view to readout of any public RSS reader, e.g. https://codebeautify.org/rssviewer

Actual results:

In version 69 and below it was shown as expected. In version 78, roughly half of the messages are lost.
What is going on is that is a torrent feed. Torrents on this feed are created in 2 forms: [1080p] and [1080p HEVC]. The majority of the content for both forms is identical, but see Expected Results for what is different.
It looks like TB 78 only accepts ONE of two messages, apparently thinking "they are similar - so they are the same".

Expected results:

All the messages must be shown.
The behavior above shouldn't be the case as:

1. all the messages on my feed have a <guid> entry, and the guids for these two items ([1080p] and [1080p HEVC]) will be different because ultimately on the 3rd-party resource it's different torrent ids and the guid is directly embedding those torrent ids. Because of this (and because it's a Primary Key on the table) there is 0 possibility that a duplicate guid will exist.
2. both the title and the body of these messages is different (though the difference is minor).

Ben, please have a look. From the de-RDF?

Yes, my fault, from the RDF-removal. The item URL is always used as the key in the feeditems.json, but some feeds should use a UUID instead. Doh.

This patch should fix it, and adds a test case.

There is the issue that this will cause duplicate items to show up for people who already have a feeditems.json using the url as key (and with feeds that use non-url guids)... I'm not sure how much of an issue this out in the wild. Maybe an uplift into 78 would head off most of the damage?

Comment on attachment 9170561 [details] [diff] [review]
1656090-use-item-id-as-feeditems-key-2.patch

[Approval Request Comment]
Regression caused by:
Occurred during the RDF removal (Bug 1534163).
User impact if declined:
Some RSS feeds will be missing items, as described in this bug (Bug 1656090).
Testing completed (on c-c, etc.):
Minor change, minimal testing (new unit test, plus superficial "it works for me" testing :- ).
Risk to taking this patch (and alternatives if risky):
Low risk, fixes an obvious regression.

Comment on attachment 9170561 [details] [diff] [review]
1656090-use-item-id-as-feeditems-key-2.patch

[Triage Comment]
Approved for beta
Approved for er78

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/89f75c0992ed
Use item.id instead of item.url as key in feeditems.json. r=mkmelin

Thunderbird 80.0b5:
https://hg.mozilla.org/releases/comm-beta/rev/1bd09bf0d762

Thunderbird 78.2.0:
https://hg.mozilla.org/releases/comm-esr78/rev/cd03df790ab7