After clicking on Delete cookies and Cache for only ONE particular website, in Cookies and Site Data, Manage Data then closing Firefox and restarting it, it lost ALL of my Tabs, previous browsing session history and my Bookmarks Setting which was pinned on the left side of the screen and the Menu Bar! I tried Refreshing Firefox but that didn't help. Under Startup, I had the box selected to Restore Previous Session. What's going on with Firefox?? This is especially frustrating because I don't have time to waste trying to fix Firefox---especially while simply trying to fix a problem with one website which as made thing worse.

Why should this problem occur and how can I get all of my previous settings back shortly and make then stay?