I don't see anything in the linked article showing that Mozilla only went with leftist outlets, much less an intention on their part to exclude non-leftist content as a matter of policy.
What I do see here is an assumption of a political agenda based on what may not even be an exhaustive list of Scroll's list of publisher sites, likely based on a single graphic on their frontpage which contains 12 logos, of which four companies were mentioned here. Their about page[0] shows 24. Are all of these "leftist" outlets? Is their only relevant commonality left-wing political ideology?
Show me some evidence of non-crazy non-leftist publishers being rejected by this partnership due to their ideology and I'll agree it's odd. Until then it just looks like confirmation bias to me.