Even if you set the setting, how can you be sure that it won't be reset on an upgrade or that you'll remember to set it if you need a new profile (perhaps your old one becomes buggy, crufty, corrupt, or all three)? I thought I had all my history retained until one day I couldn't find a website I knew I had visited years ago, and took a closer look at my history and was very unpleasantly surprised... What happened? I'll never know, but my suspicion is that Firefox reset the history retention setting at some point along the way. If you do any web dev, you know Firefox occasionally backstabs you and changes on updates. The only way to be sure over a decade-plus is to regularly export to a safe text file where the Mozilla devs can't mess with it. I can't undo my history loss, but I do know I have lost little history since.