In Firefox , currently, using the Forget button causes all pinned tabs to be closed (regardless of the timeframe of data to 'forget'). I think this is an oversight. I think either:
There should be a toggle to choose to either preserve pinned tabs or close them
Pinned tabs should automatically be preserved
There should be some kind of warning in the help article or inside the text box when opening the Forget button itself that pinned tabs will be closed as well. Currently this isn't entirely clear, and could cause some confusion.