The installation went without problems, update to Plasma 5.26.1 too.
The sound at the start of the system is back.
Main remaining problem, the connection between Thunderbird and Firefox.
Clicking on an HTML link in Thunderbird opens a Firefox session, but the HTML page is not display.
This happens with the Deb version of Firefox. By installing the Snap version the problem is solved. Both versions break the plasma-browser-intégration.
SM

since a few minutes ago I have:
lsb_release -a
Distributor ID: Neon
Description: KDE neon 5.26
Release: 22.04
Codename: jammy

The upgrade on a MacBookPro 12.1 went smoothly. Moreover this time the connection between Thunderbird and Firefox is perfect (Deb package of Firefox), the plasma-browser-integration is OK.
Where could the problem come from during the installation ?
Thank you for your suggestions
SM