Do you open tabs and forget to close them later? That means you’re likely to end up with a whole bunch of tabs open with no clue as to what to do with them. What you need to do is to click that little cross on a tab and learn to say good-bye. Alternatively, you can save several joules of energy by using Tab Replace, a Firefox extension that opens bookmarks in currently open tabs instead of opening new ones. While this doesn’t decrease the number of tabs you might have open, it does make better use of them and does not open new ones to add to the clutter.
The extension adds a new option to Firefox’s options; go to Tools > Options and open the Tabs Tab. Check When opening multiple links at once, replace existing tabs.
To open bookmarks in your currently open tabs, go to the bookmarks manager, select the bookmarks you want to open and hit enter. There will be no additional option available to open the bookmarks in the current tab; the extension will overwrite the default open in new tab function so that tabs opened from the bookmarks manager always replace the currently open ones. The extension opens tabs in the last active window but does not let you specify which tabs to overwrite.