Firefox opens the bookmark manager (which is officially called library) in a new window by default. I always felt that this was kinda weird as it meant to navigate back and forth between the web browser window and the bookmark manager. One could try to place the two windows next to each other or display the bookmarks in the sidebar instead but it always nagged me why there was not an option to display the bookmark manager in a tab instead of a window.
Places in Tabs changes this. It is an experimental Firefox add-on that will add the option to launch the bookmark manager in a tab in the Bookmarks menu. The library is then displayed in a new tab in the active web browser window. Users who want to work with the bookmark manager that way need to know that a left-click on any bookmark will load that website in the active tab. To keep the bookmarks manager open users need to middle-click (or right-click and select Open in a new tab) instead.
Places in Tabs might be interesting for users who would like to open the bookmarks manager in a tab. The add-on is compatible with Firefox 3 only and can be downloaded from the Mozilla website.
Update: Places In Tabs has been discontinued. There does not seem to be an alternative for the feature at this point in time.
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