Not to sure. But give this a try : From the Menu Bar Toolbar -- > Click Bookmarks --> Show All Bookmarks --> Views --> Sort = Default is Unsorted. See if it has been changed. Or rearrange how you want.

Alternatively Open Bookmarks and right click and on a folder/s and then Click Sort By Name.

Please let us know if this solved your issue or if need further assistance.

Are you using Sync?

If so, Disconnect ... from Sync on the device that you want to rearrange your bookmarks on before moving any bookmarks around. Once your changes are completed, close Firefox and re-open Firefox - then Connect ... to Sync again.

If that jumbling re-occurs use Verify Integrity for Places Database at the bottom of about:support.

I turned off Sync and the bookmarks stayed put. They were never jumbled horribly, just moved around some and when I moved them back they moved around again. I have not turned Sync back on though. Should I?

"... when I moved them back they moved around again."

When you closed and then reopened Firefox? Or they moved all by themselves while Firefox was running?

I first noticed it after Firefox updated the other day. I would put them back where I wanted them and then a little while later they would jump back to where they had been jumbled to. I want to say anytime I opened a new window or tab they would jump.