Firefox sync not working - tried all the standard troubleshooting steps

I'm having trouble getting firefox sync to work with my mobile phone. I've tried all the usual troubleshooting steps - the only thing that "works" is completely uninstalling the Firefox For Android app, reinstalling it and signing-in as part of the initial set up. This syncs my history and bookmarks during that initial set up process. However, after that it never syncs again. When I press 'Sync Now' to force a sync, nothing happens. If I sign out and sign back in, I get a message "Your account needs to be verified. Tap to resend verification email". Tap that and I get taken to the about:accounts homepage and everything looks ok, but it still won't sync and when I go back onto the sync options page the warning message is still there. I don't receive a verification email (my firefox account was created and verified several years ago). Firefox 68.0