The Firefox extension is bit hit and miss, the exporting function has been broken for years (no points for guessing why that might be). I am not that happy with it and I do not recommend it. I used to pay for the access to their Android app but that became a free feature ages ago; shortly after the company changed hands, I think. The premium subscription basically gives you some online storage and better support but I don't think those features are worth the subscription cost and so I downgraded to the free tier.