Could you be more specific? It would be helpful for you to post a screenshot of your problem. Press Print Screen on your keyboard, then open up Paint from your start menu and go to File > Paste. Save the image somewhere, then attach it to a reply here. That will help us to better diagnose your problem.

When you save the screenshot as am image, you need to save as a JPG or PNG image - Paint defaults to saving as a BMP file, unless you specify another format. https://support.mozilla.com/en-US/kb/Adding+screenshots --- It sounds to me like you have that "personalized start page" opening in the Sidebar, instead of a true Tab in the main browser window. A screenshot will tell us.

Opening in the side bar 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 side bar (Ctrl+B).
In the Bookmarks Manager (Bookmarks > Organize Bookmarks) you can click the More button in the Details pane at the bottom right.
Make sure that "Load this bookmark in the side bar" is not selected.

Thank you