I pinned a number of web sites using Firefox with latest updates. When I opened Firefox the next time there were nothing pinned. I may have used CCleaner before shutting down,could this have caused the problem?
Go into Ccleaner and under 'Applications' look at what cleaning you have selected for Firefox. Unticking these will stop the removal, but will also prevent any cleaning so you may wish a bit of trial as to which actually removes your pinned sites.