This has happened a few times and confused me but I figured it out tonight. I had a private window open and a regular window open and closed the regular one. When I reopened the regular one all of my pinned tabs were gone. This should be fixed, Firefox should differentiate between the windows and stop closing tabs on the regular window if it's closed before the private one. Kind of an odd thing.

I was able to recover my tabs with a json file thankfully but haven't been able to do so before as I had opened and closed multiple windows trying to figure out where the pinned tabs had gone and the "previous.json" had changed to those new open tabs. It might also be nice to have a few "previous.json" files be created instead of the single one for instances like this.