Make sure you do not drag a tab slightly down in the browser window while clicking the tab. Current Firefox versions have a feature called tear-off tabs. You can detach a tab from the current window and open it in a new window by dragging a tab in the browser window. You can also do this via the right-click context menu of a tab: Move to New Window You can drag that tab back to the tab bar in the original window to undo this detaching.

Yeah that is the problem. I switch from one pinned tab back to the main pinned tab and it pulls off. Trying to 'be careful' is not a helpful solution because obviously I am not attempting to do this, and I have had my workflow set up exactly this way for years, and with previous versions of firefox I did not have this problem. Now all of a sudden it is happening over a dozen times a day!

The other problem is that when I drag the tab back to the original position the input fields in the browser become stuck so that I can no longer enter my cursor into any fields and as a result I have to completly close the tab and then reopen the tab for the input fields to work again.

So this is not a minor inconvenience. When the tab gets pulled it freezes up the browser so that no inputs can be put into any fields.

So again, how do I LOCK the tab so it can not be pulled off?

There used to be some extensions that prevented this issue, but that are Legacy extensions that no longer work in Firefox 57+ and some have been removed from the add-ons website. This means that there is no way to guard a tab against accidentally tearing off.