If you are signed into Firefox, and you have multiple devices, like a desktop PC, a laptop, a raspberry pi, and all of them have Firefox-beta and Firefox-developer-edition, deleting history isn't a child's play, it can hours of work! For example, A month ago, I deleted the older histories from my laptop. If you delete many histories, Firefox will slowly consume all your system's available RAM unless you kill it, ok fine, probably Firefox is written the inefficient way. Then I turned on my raspberry Pi. And all my hard-work of deleting history got synced with my laptop. Ok I deleted them again. Then I turned on my Computer... Oh No! Again it got synced... They say patience is the key to success (here success is deleting histories). Then I realized that I had a tonne of virtual machines, I use them once a month maybe. Executing Firefox on them would sync them again or so. So I had to put myself in a state of meditation to remind my subconscious mind to remember to delete the ~/.mozilla/ directory whenever I open my virtual machines. I am a mere human, I might not have patience like the Firefox devs, I can't waste months deleting histories, I have other works to do. So I went to the Firefox sign in page, signed in, disconnected all the devices, removed the ~/.mozilla/ directory, then logged in again, synced, deleted histories, and then slowly I singed in to all the devices. In Firefox, forgiving your past is not that easy. So what do you think is it a child's play to delete your histories?