Open Menu(at right top) of browser and select customize from the bottom. Now Drag bookmark bar from Browser head where tools /options are resided. Click on Exit Customize . Now you have a Bookmark toolbar items in your stack. Open it in full screen.

PS: If bookmark bar is not visible on browser head then right click on head and check mark bookmark bar from list. How to : https://www.mozilla.org/en-US/firefox/desktop/customize/

Could be a little known feature of the bookmarks... try this:

right-click the bookmark, then click Properties. If the "Load this bookmark in the sidebar" box at the bottom is checked, uncheck it.

Usually that is checked when you let a website add the bookmark for you instead of creating it using the built-in features.

In the Bookmarks Manager (Bookmarks > Show All Bookmarks) you can click the More button in the Details pane at the bottom right. Make sure that "Load this bookmark in the sidebar" is not selected.

Note that opening in the sidebar is the default for bookmarks that are created via a link or button on a website. You can check the properties of a bookmark via the right-click context menu in the sidebar (Ctrl+B; Command+B on Mac).