Make sure that you aren't using a bookmark that open is 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.

I have a hundred-plus bookmarks, so checking each one to see if "Load this bookmark in the side bar" is selected would be a royal pain. Any way to do this more efficiently?

BTW, the I loaded FF in safemode, and the problem stopped, so it's an extension causing the problem. Any way to find out which one, other than disabling one at at time?

Thanks

If you have many extensions then first enable half of the extensions to test which half has the problem.
Continue to divide the bad half that still has the issue until you find which one is causing it.

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Adblock was causing the problem. Now, I have to figure out why. Thanks.