Harper Firefox extension - "You need an updated version of Firefox for this extension"

I don't know if this is expected for some extensions but, when I try and install the Firefox version of https://addons.mozilla.org/en-US/firefox/addon/private-grammar-checker-harper/, I get told I need to update Firefox.

Previous reports about this happening have been closed (Raindrop.io & Enhancer for YouTube) so maybe it's extension specific?

I expected to see an "Add to Orion" button, the same as other extensions I've tried to install from the Firefox site.

Orion Version 0.99.134-beta (WebKit 622.1.9.19.2) Build date June 10, 2025 Mac 15.5 (Build 24F74)

Sequoia (15)

I ran into this, a simple workaround is to click on the "Download file" link. That will start the install process.