From a quick web search, while it supports installing firefox plugins they may not all work as expected due to limitations/differences in the browser APIs, and ublock origin fails to work properly as a result.

So IMHO mozilla are probably choosing "Let's not" rather than "Let's deliver a broken experience"

As one user on reddit said - "Orion supports Firefox and Chrome extensions. Yes. And you can install them. Yes. Do they work, though? No, most of them don't. Orion is lying about their extension support. "