When I look at this one, there are only 7 items in the list from the server:

http://dilbert.com/feed

(Same if I open it in Chrome.)

Is it possible you were using a different one before? Maybe they changed it...

If I understand correctly, the number of days would be defined by the site.

It makes sense, and nothing the user can do about it.

Thanks

A web feed XML file only contains data about all included items and Firefox processes this file to show all feed items in a folder list, so yes solely the website determines what feed item are shown. When the website creates a new feed file then they can decide to remove older items that no longer apply.