Make sure that you aren't opening pages in the sidebar.

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; Cmd+B on Mac).
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 side bar" is not selected.

Thank you Cor-el for your quick reply. Unfortunately the problem is unsolved. I checked in Properties and none of the bookmarks had Load in Sidebar checked. I tested, by checking Load in Sidebar, [which gave me a sidebar in addition to what was already on the screen] logging out and then unchecking it, logging out and opening it again, unchecked. No luck. Still only left 1/3 of page. I tested by removing the bookmarks, logging out, then reinstalling the bookmarks. Unchanged. Same problem. With the sites on the Search bar on the right of the address bar, Amazon.com is also screwed up, but not all the sites: Wikipedia, You Tube are fine. Have you any other suggestions that I could try? Thanks, Miranda ps. Incidentally in my Firefox 5 and Windows 7 setup, there is no "more" button on the Show All Bookmarks Screen. I got to Properties only by rightclicking each site's tab.

Dear Cor-el, Well, I fixed it! The trouble is that I don't know how I did. I just went through step after step in Basic Troubleshooting, reinstalled Java (which was the last thing updated before the disaster) and generally clicked on every button in sight. Finally I shut down, went for a walk, rebooted and all was well. I suspect clearing the cache and the new Java were part of it, but I honestly don't know. Thanks for your concern, Miranda

You're welcome.

Such issues can also be caused by zooming a page or increasing the default minimum font size.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)
• Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"

Reset the page zoom on pages that cause problems: View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

Thanks, again. I especially welcome your clear and precise instructions that even computer idiots like me can understand. M

Please click the Solved It button next to the answer that solved your Firefox support issue, it appears when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.