For more than ten years I'm the developer of mailmindr, the free Thunderbird extension for setting reminders and follow ups. I'm doing this in my free time, so if mailmindr is helpful for you please feel free to send me a donation.
If you want to support my work on a monthly basis, there's a Patreon site for doing so. On https://patreon.com/arndissler there are some tiers available. Pick the one which suits best.
You want to send a one-time donation? PayPal could be a good choice for this: https://paypal.me/mailmindr.
There's an amazon wishlist with some items I find useful, either for me or my kids: https://www.amazon.de/registry/wishlist/WHZ3626BZ2C7.