Agreed. I hastily installed FF Preview and then immediately uninstalled it when I couldn't find where to install uBlock. They shouldn't have launched, or made an announcement, without some form of adblocking (native or plugin).
I understand that it's a testing release, but they stand to lose out on many testers and users. While I am perfectly willing to be a guinea pig and use this as my daily driver, I am passionately unwilling to browse with ads (especially on a mobile device where data is more expensive).
I am likely part of the vast majority of potential testers/users. There probably won't be a "hey, we support plugins/blocking now" announcement and, even if there will be, it probably won't be as visible as this announcement.
Did you actually try it on a website with lots of ads? I've been using focus for the last 6 months or so and the tracker blocking also blocks many ads for me. Also there is blockada which blocks ads system wide.
Winning their userbase back is the only way to win back the leverage they once had...