The extension is available for Firefox
Save to Screenly is available as a Firefox add-on, so you can add web pages to Screenly from Firefox as well as Chrome.
The dashboard you want on the lobby screen is open in a Firefox tab. Now you can send it to Screenly from that tab.
Save to Screenly is on Firefox Add-ons, alongside the Chrome Web Store version. Install it, sign in with your Screenly API token, and it works the way it does in Chrome: open the page, select the Screenly icon in the toolbar, and select "Add to Screenly".
The Firefox add-on has the same options as the Chrome extension, including "Save Authentication" for pages that need you to be signed in.