Clipper not working in Firefox for Mac on Event Pages (whatever that is)

[AyushSharma 2]

Hi.

I'm trying to clip a few pages in Firefox for Mac and its not working. The clipper window either doesn't open or it shows a shadowy arrow and then nothing. In Firefox add-on debugging I keep seeing this:
 

Evernote Web Clipper

    Reading manifest: Error processing background.persistent: Event pages are not currently supported. This will run as a persistent background page.
    Reading manifest: Error processing options_page: An unexpected property was found in the WebExtension manifest.

Extension ID
    {E0B8C461-F8FB-49b4-8373-FE32E9252800}
Internal UUID
    3f108101-b80b-8a4e-a6fa-af4b83677b81Manifest URL

I can see that the clipper is not working with pages that have a manifest file. Is that a progressive webapps thing? I'm not sure. I tried this on other sites that don't have manifest files and its working.

I'm using Firefox 56 (64-bit) on Mac Sierra (10.12.6). Evernote Clipper version is 6.12.4, which is ready for Firefox 57, but not 56, apparently.

The page in question is getting clipped in Google Chrome, but the "Clipping..." window is not there. As soon as I hit the Clip button, the window disappears, but the note eventually shows up in my Evernote account.

What do I do?