I use 2 (or more) devices. I have noticed that when I clear my history on one, it still exists on the other. This is not helpful as there is a reason for clearing history. It's weird that the history is shared without my knowledge (until now) with other devices I use but maybe it's a shared one. When history is shared with different devices it should either be deleted on all devices and if this is not possible it shouldn't be shared between devices. Firefox doesn't know if one is used private and the other used for work. To clear history is actually a recommendation by Firefox to reduce garbage or tracking abilities or what ever. It seems to me that Firefox uses computer identification (as the web does)
My suggestion is not to share unless requested.