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 > Organize 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.

@cor-el Thank You, Thank, Thank You!!! I never thought to right-click the bookmark. Personally, I think it's insane to have them open in the sidebar. Why on earth would that be the default? Sounds like change for the sake of change to me. Anyway, thanks for the tip.

No "change" involved, it has always been that way with Firefox.

Uh, not quite. I've NEVER had to uncheck a default that opened a bookmark in the sidebar. NEVER. In fact, no bookmark ever opened in a sidebar.

You're welcome.

It depends on how you create bookmarks. It won't happen if you click the star on the location bar or use other Firefox means (Bookmarks Menu or right-click context menu) to create bookmarks. It only happens if you click a button (link) on a web page to bookmark that page.

absolutely NOT true. This never happened until a year or so ago. What you're implying is that users suddenly changed the way they create bookmarks. I ALWAYS use Ctl-D to create bookmarks. Firefox changed. NOT us. Wrong-o-rama.

The problem was happening to me as well, for one particular site... it was driving me nuts. I tried viewing the page source, but "View page source" only works on what's in the main browser window, it does nothing for the Sidebar. I even went so far as to run Firefox in Safe Mode to see if a plugin was causing it. Thankfully, I found this thread.

I don't understand this behavior. Why on Earth would anyone want to ever open a bookmark in the Sidebar??? Who came up with this "feature"? They ought to be slapped!