Some software is so complex and difficult that Debian does not maintain it on their own, and instead follows the upstream release cycle.
Browsers are one such example, and as you've discovered for me, Thunderbird is probably another.
Also, please do not recommend testing for daily usage. It does not receive critical security updates in a timely manner, including for things that would effect desktop users. Use stable, Sid, or another distro. Testing is for testing Debian ONLY, and by using Debian Testing, you are losing the advantage of immediate security fixes that come from literally any other distro.
Maybe not some obscure ones, but here are some lesser known ones:
Talos Linux. It's an immutable operating system designed specifically to deploy kubernetes.
OpenSuse Harvester Think Proxmox, but instead of VM's and LXC containers, it's VM's and Kubernetes.
XCP-NG is a RHEL based distro designed for managing Linux virtual machines using the xen hypervisor, as opposed to KVM. Think Proxmox, but RHEL and Xen (also no LXC). However, it does not come with a web ui out of the box, you have to deploy it yourself. Technically, XCP is a Xen distribution, since Xen is a kernel with nothing but a hypervisor that runs under the main distro, but the primary management virtual machine is RHEL based, and uses Linux.
Speaking of Proxmox, Proxmox is technically a Linux distro.
SnowflakeOS is a project that aims to bring a GUI focused experience to NixOS.
TurnkeyLinux (site is loading very, very slowly for me right now) is not a single distribution, but rather a set of debian based distributions that are designed to be turnkey appliance virtual machines that contain and host a specific app. To deploy the app, all you have to do is set up the virtual machine.
Now, here are some not-linux, but interesting distros:
SmartOS. They ported KVM to unix, and also can use Linux syscall translation (similar to wine) to run apps in containers as well. There is also Bhyve. It's a very interesting hypervisor platform.
OmniOS is similar. Bhyve, KVM, and Linux syscall translation in containers.