From MozillaZine Knowledge Base
When loading a group of bookmarks at once (for instance, middle-clicking on a bookmark folder), 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 Firefox preference allows the user to pick a behavior for loading bookmark groups: appending the bookmarks as new tabs, or replacing the existing tabs. This option was removed in Firefox 3 alpha 8.
Possible values and their effects
Replace all existing tabs with new tabs for the bookmarks in the bookmark folder. (Default)
Append new tabs for bookmarks in bookmark folders to the exsting tab bar.
Mozilla Phoenix (0.3–0.5)
A checkbox labeled “Replace the current set of tabs” is located under “Tools → Preferences → Advanced → Tabbed Browsing → When opening a bookmark folder in tabs”.
First checked in
Has an effect in