Opening all unread item links on new tabs

The number of tabs opened is limited to a maximum of 15.

There is no user interface to set the maximum number of tabs, and this has been done on purpose.

If you want to override the maximum number of tabs, you can create an integer pref named WizzRSSMaxTabs
and set its value to the maximum number of tabs you'd like to open.
If you are able to create a Firefox pref, you are also likely to understand the ramifications of opening too many tabs.