Mozilla Firefox

I have started using Mozilla Firefox, and like it. I started using it because everyone kept telling me that it far more "secure" than Internet Explorer; and that when they went to Firefox, they had little or no problems with viruses, etc. and adware. But I have had some problems which probably happen to everyone trying to acclimate to a new system. One of the most puzzling is that every now and then I get a huge space at the bottom of my screen. I find that when I go to HISTORY and click on "show in Sidebar," I will, in fact get a sidebar on the left side of my screen. This seems to control the space at the bottom - but I am sometimes concerned that I might be doing something wrong. I do know that when I click on the "tab" at the top of the Sidebar, the space at the bottom disappears. What is happening? Just what is the function or benefit of the tabs? Sometimes when I have a page filled with answers from a search, the tab prevents me from going back to the page of search answers (then again, maybe I am just clicking on the wrong buttons).