My build (just downloaded as a fresh install) is #20351804. Add-ons are a menu option now, with UBlock Origin as the only recommended one. Actually, it might be the only one available at the moment?
Judging by the previous topics about Firefox Preview, uBlock is by far the most requested one.
Worth pointing out that Firefox Preview is set to replace current Firefox within the next 5 months, add-on support will probably arrive a version or two after that.
"Full add-on support will be available in late 2020." "The Firefox for Android Beta and general release migrations will follow and the final migration will occur in Spring 2020."
Mozilla were cagey about this for a while, but this doesn't come as a surprise to any of the many people who predicted they would do this.
Worth pointing out that Firefox Preview is set to replace current Firefox within the next 5 months, add-on support will probably arrive a version or two after that.