I checked again today on my Firefox 60.0.2 (64-bit) Mac OS and it works but only if you're signed in to Google. Try opening another tab, sign in to Gmail for example, then go back to the tab you wanted to highlight and reload the page. Then you should be able to highlight that page. Then it should work on all new pages you visit (but reload the tabs you had opened before you signed in, because Yawas uses the presence of a cookie to know if it can store your highlights in Google Bookmarks).