When I pin a tab, it immediately disappears; how do I find it?

When you pin a tab, it go to top left of the window:

Read more here:

• Pin Tab

I must be doing something wrong. There is nothing in my upper left corner regarding pinned tabs that I can find. When I then close Firefox and reopen it, there is still nothing there and the tabs I pinned do not seem to show up anywhere.

Try pin a tab in Safe Mode:

• Firefox in safe mode
• Pinned Tabs

If you use one of the keyboard shortcuts Ctrl 1 to Ctrl 8 to reach the first eight tabs does that work?

You can try to open a new window to see if it work better in that new window.