This is working great. This lets me access my all my Google Bookmarks and my Read Later list.

2nd edit:
To fix the problem of this not being logged in to one's Google account, clear Firefox data (cookies, cache, & active logins) and then go to a Google login-in page.

1st Edit: After some digging I found this out:
From browser console:
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://accounts.google.com/ServiceLogin?continue=http://www.google.com/bookmarks/...
...
(Reason: CORS header ‘Access-Control-Allow-Origin’ missing)

These pages might shed light on the problem:
https://stackoverflow.com/questions/24371734/firefox-cors-request-giving-cross-origin-request-blocked-despite-headers#24383474
https://support.mozilla.org/en-US/questions/1152971