That is a replacement for the 'open all in tabs' functionality lost in FF13. Prior to that each live feed had this option within its own menu.
It requires the following setting in order to work properly:
in Windows: Tools>Options>Applications>Web Feed = "Preview in Firefox"
in Linux: Edit>Preferences>Applications>Web Feed = "Preview in Firefox"
On click on the icon in the add-on bar it finds existing RSS live stream subscriptions and lists them in a panel. If no live feed subscription is found the panel will report so, and ask you to create one first.
On click of one of the live feeds listed you will get an expanded menu listing all feed items and on top a menu item 'open all in tabs'. On click all listed items will open simultaneously. Careful - opening all items at the same time may significantly slow down your browser. Some feeds include videos, which then may start playing all at the same time (to avoid this you could e.g. install the flashblock add-on).