From MozillaZine Knowledge Base
When opening a bookmark group, the default behavior is to replace the window’s current tabs with new tabs for the bookmarked sites. This may seem to be counterintuitive behavior, but according to Peter Annema, a (never cited) usability study indicated opening additional tabs from bookmark groups was considered more confusing. This Mozilla Suite preference allows the user to pick a behavior for loading bookmark groups: appending the bookmarks as new tabs, or replacing the existing tabs.
Possible values and their effects
Append new tabs for bookmark groups to the exsting tab bar.
Replace all existing tabs with new tabs for the URLs in the bookmark group. (Default)
Radio buttons labeled “Add tabs” and “Replace existing tabs” are located under “Edit → Preferences → Navigator → Tabbed Browsing → When opening a bookmark group”.
First checked in
Has an effect in