I installed Ubuntu 24.04 on my Dell desktop. This went well, but after installation I noticed the same problem with Thunderbird that I had seen on an Ubuntu upgrade on an old HP desktop. When I wrote before about not seeing the issue on a fresh installation it probably meant my testing was inadequate. This is a big problem: I am not willing to give up my multi-level email folders.

So I made clean installs of Debian 12 on both the HP desktop (with the upgraded Ubuntu) and the Dell (with the fresh install. The installations went well, with one glaring error — see below. Thunderbird works just fine on both, with no problems with the multi-level email folders.

In previous installations of Debian 12 I had used the net installation iso, debian-12.6.0-amd64-netinst.iso. This time I used the full debian-12.6.0-amd64-DVD-1.iso, which does not need to download files from the internet on both desktops. In both cases the installation failed at the step of installing the grub bootloader. This is often fatal to the installation process, and never happened to me with the netinst version. What I did here was skip that step, after trying to rerun it without success. When the installation finished I rebooted. The old version of grub, from my Ubuntu installions, was still present. So I booted into Ubuntu and then ran sudo update-grub. This worked and found the Debian partitions. I rebooted again and Debian was then available in the boot menu. After booting Debian on both systems I went through my usual Linux configuration process and additional steps from Debian Notes

Right now I am using Debian as my daily driver, but I will use the netinst iso for future installations. If I can find a way around that email folder problem I may give Ubuntu another chance. Ubuntu’s installation process is smoother and there is less tweaking to do after installation.