I recently lost 150+ tabs on Mozilla Firefox because I wanted to remove my last 5 minutes of browsing from my history. I have NO IDEA why the tabs I opened 3 months ago would be closed by this, and not be under "Recently Closed Tabs". Or why there's no backup file from 5 minutes ago I can restore from, even if it means creating a files when I "forget 5 minutes", then delete the file after a day or something. It would cost nothing in space and processing power, and would prevent such accidents.
Currently, I rate the Forget Button "Useless/10".