Since no interaction with other addons is involved, and sincere-installing Firefox doesn’t help, my problem lies in either my system or the addon itself.
Not necessarily. I assume you haven’t cleaned the Firefox profile… IIRC, it’s not cleaned on reinstall. BTW, you can try testing a new profile.
Steps 1,2 and 3 are fine
If step 3 is ok (feeds do reorder in the Brief window), Brief livemarks reading is (at least partially) working.
step 4 doesn’t work
So, even when using the Live Bookmarks, the live bookmark is not created in the folder you select for it? Sounds like a Firefox bug as Brief is not involved in this operation at all.
Is there anything in the error log on steps 4 or 5? You can open the error log with Ctrl+Shift+J, clear it (button in the top left corner) and try subscribing (either via Live Bookmarks or via Brief).