As the tooltip for "Feed update threads" select box says, it defines the maximum number of concurrent threads when updating feeds. Thus the scenario you describe is not possible.

If you open the feed in a normal browser tab, you can see that the XML content contains only the latest 10 articles. Feedbro can't initially load more articles than the feed provides. Later when new articles are published, the old ones are preserved until the "Max entries" number is reached. After that the oldest article is removed.

If you prefer the popup view, then there's the "Open Explorer" menu which will show the folder structure.

This is caused by a recent regression bug in Firefox. See https://bugzilla.mozilla.org/show_bug.cgi?id=1535068

We'll see if there's a way to get around this somehow but the problem is that it works fine on Windows but not on Linux or MacOS.

Workaround: open about:config and set "ui.context_menus.after_mouseup" to true.

Please see Feedbro help for a list of keyboard shortcuts. You can open the Help by clicking the ?-icon in the Feedbro toolbar. You can use j/k or shift-rightarrow/leftarrow to move between next/previous item on the list.
Page down/up and space bar affect the actual article view in viewmode 3 as intended.