Q:

How to merge multiple RSS feeds into one, in Firefox?

I have a bunch of RSS feeds in Firefox. If you've ever subscribed to one, you'll know it creates what looks like a folder, which upon hovering, pops out to show the contents of the feed.
I want to have it so that I simply have one "folder" on my bookmarks tool-bar, which, which expanded, shows them all in one - as in, if I had 3 feeds I was subscribed to when I click on the folder it will show:
[]
[] Name of content on site 1
[] Name of content on site 2
[] Name on content of site 2
[] Name of content on site 3                  *in chronological order*
[] Name of content on site 4
[] Name of content on site 1

Is this possible? I guess the only way it could work is if RSS feeds have timestamps, do they? I've practically no knowledge of how they work.
If it does turn out to be impossible in firefox, what's the lowest footprint software that can accomplish it?

A:

I just use FEEDcombine, you enter in all of the desired feeds, then it gathers them for you and gives you a combined feed link you can bookmark.
Here's an example of StackOverflow and SuperUser feeds combined:

