Hello to fix this follow the steps below.

1. right click the bookmark and select properties.
2. Uncheck "Load this bookmark in the sidebar"

Hey there,

I've removed your email address from the title of this post. There are some not-very-nice robots that browse the internet to look for email addresses, so they can send junk mail to you.

Many thanks. But what in the world caused caused the bookmarks to gain the property checked to open in the bookmarks column?

Hi benorr, here's a guess. Some websites offer a link to bookmark themselves. Firefox doesn't allow sites to add normal bookmarks, but they can add a bookmark that opens in the sidebar. It has been that way for so long, I can't remember the reason...

I do not understand how most (but not all) of the bookmarks suddenly had the "load this bookmark in the sidebar" checked. Many have been used every day on this computer for a year or more, and until yesterday, never acted this way. It is a puzzlement, at least to me. Thanks for the fix (and the suggestion not to post the email address publicly), but I am still wondering what caused the change.

It could be a sign of creeping file corruption affecting your places.sqlite database, which stores your bookmarks and history. If you remove this file, Firefox will rebuild it from a backup of your bookmarks. However, the most recent backup(s) may include the "open in sidebar" designation already. So that might not be a good idea.

I'm not sure what to suggest at this point, except saving copies of your most recent backup files in a safe place may be helpful. You can copy them from the following folder:

Use Help > Troubleshooting Information > "Show Folder" button to open your currently active Firefox personal settings folder (also known as your Profile folder).

Double-click the bookmarkbackups folder. You should see several dated backup files.

Copy them to a safe place (e.g., Documents folder, USB flash drive, Dropbox) in case you need them for future restoration.