Firefox is also moving to Manifest V3, but a more "relaxed" version that still allows a lot of what is being removed in Chrome.
What seems to have happened here, is that uBO decided that, since they now have a declarative version for Chrome, they may as well release it for FF also (but with a few improvements, apparently).
I think it's a smart move on FF's part (a more relaxed V3). If Chrome goes too heavy-handed with anti-adblockers in V3, people might leave Chrome. Yes, they might leave Chrome for a different Chromium browser, but if they get too heavy-handed there, Firefox and its forks are the last large alternative left.
What seems to have happened here, is that uBO decided that, since they now have a declarative version for Chrome, they may as well release it for FF also (but with a few improvements, apparently).