Hello Nonya,
Regarding the "long term stability" - I'm using my own addon every day and I can't imagine my Firefox without it. So as long as I live, I will make sure it works, including the server. I'm only 33 years old now and I've been working on it for 5 years now and I see myself working on it 5 years from now and more :).

There was a one-time payment in the beginning (years ago), but it wasn't generating enough money for me to survive and sadly even now when check my Patreon page, I'm not making a lot (this is my full time job).

Regarding privacy concerns, I will be adding custom server synchronization (sometime in the future) and also End to end encryption (this should come in Q1 2022). But for now I can tell you that nobody except me has access to users data and I'm doing my best to protect it.

Anyway, if you have questions, feel free to contact me to my support email. And even though I can't offer you Pro version without the cloud account, I may give you some nice discount if you can't afford it right now (I will be adding better pricing once I finish my server upgrade).
Also I'm sorry for the delay, I'm a bit sick so I'm in bed most of the day :(

Best regards,
Juraj Mäsiar