When I select one of my bookmark sites, it opens is a tiny sidebar on the left of my home screen tab. It has been opening in a new tab with full view of the site I had bookmarked.
My home page is MSN. From the menu I select bookmarks, choose the one I want, and it opens, but only in a tiny sidebar to the left, on the MSN home page. It just started doing it. It has been opening in full view to the bookmark page.
Opaite Mbohovái (1)
Right-click that bookmark and select Properties, then un-check the preference for Load this bookmark in the sidebar, and then click Save.
Using JavaScript buttons on web pages for "Bookmark this Page" causes that preference to be selected when saving the bookmark in that manner. So, don't use those buttons, use the "Bookmark this Page" feature that is built into Firefox instead.