Sure (kind of)! Make a donation to the Mozilla foundation. https://donate.mozilla.org/ It's not directly paying for the product, but it also does show as "this user wants our vpn" in the stats.
As always, a note needs to be attached to this link. Money donated to the Mozilla foundation goes exclusively to their advocacy work, and none goes to the Mozilla corporation, which develops Firefox.
I reckon because it's not possible to pay for the product directly, Firefox is getting less financial support than it could. For example, I may not want to support many of the goals of the Mozilla foundation, but paying for a product that I'm using (Firefox) sounds reasonable.