I'm pretty sure all browser extensions are "open source" but I get what you're saying.
That qualifies as Open Source under any reasonable definition of the word [1]. So being Open Source is no argument for or against either Adblock Plus or ublock origin, because both are.
Then you have a gazillion of "Open source licenses", that can be less "free" to more "free", like BSD, GNU, MIT, Apache, the free source license, ...