First of all, Firefox Preview (code name Fenix) is Mozilla's effort to create a new mobile browser, so this is unrelated to which version of Firefox you run on your desktop.

Second, it's called Preview for a reason. Support for add-ons is currently being implemented, so if you install Firefox Preview on an Android device right now, you won't be able to use uBlock Origin yet.

Third, the built-in tracking protection blocks most ads (at least on websites I frequent EDIT: which do not include Google or Facebook etc.), so you can already try it out if you just want to see fewer ads and don't absolutely need uBlock Origin.