My Firefox boomarks sidebar has changed from full-screen to half screen length... Why/how?

My bookmarks bar has always been full length of the screen with a scroll bar to the right. A few minuted ago when I clicked on the bookmarks button (the star in the top left) the sidebar is now half the length (still has a scrollbar down the right hand side).



This has happened once before and at the next boot-up it was back to a full length screen bar.



Since I haven't altered any options, and have been just surfing a regular news site today, what has happened, and why???My Firefox boomarks sidebar has changed from full-screen to half screen length... Why/how?
I think what you are seeing is the Tabs. You can have multible webpages running at once...each website will have it's own tab. You can close each tab by clicking on the X just to the right of the ';star'; or other...depending on what that page displays.



Cheers

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