I think he means Firefox is not Chromium-based; Chrome is a proprietary code base, no one can access it besides internal Google employees. Chromium, on the other hand, is open source, and is what browsers such as Edge, Brave and Opera are based off of. Chrome is also based off Chromium, it’s the closed source browser Google distributes. Think of Chromium as Android, and the Pixel UI as Chrome. So no, I doubt he means Firefox is Chrome.

Edit: seeing from the other comments, downvotes, and that the comment has been edited, he most likely made a typo and typed “Firefox is chromium.”

Edit 2: Mistakenly said that Chromium was what Firefox was based off of when I meant to say Brave. My bad. I’m well aware Firefox isn’t Chromium-based(I use it for that reason), I was just confused as to why this person was saying he meant Firefox was Chrome, when the comment read “firefox is not chromium”. I later realized(in the above edit) that they must have written “firefox is chromium”, before editing it to “firefox is not chromium” after realizing he messed the comment up.