But Firefox is so dependent on google (money, code) that it's absolutely impossible they won't also remove manifest v2. It will just take a little while, for appearances...
About a year ago FF said they had no current plans to remove V2 support, and if they did, they'd give at least 12 month notice. Which to me is basically language saying they absolutely will remove it at some point, otherwise they'd just say "no we'll never remove it, fuck google".