What worked for me (answering my own question): I went into /etc/apt/sources.list and copied the first line:
deb http://gb.archive.ubuntu.com/ubuntu/ jammy main restricted
and rewrote it for Kinetic, I added the following line, I did not remove the Jammy line:
deb http://gb.archive.ubuntu.com/ubuntu/ kinetic main restricted
Then I ran sudo apt update, it took a longer time than normal and reported thousands of updates. I only ran the updates that Firefox and Thunderbird had wanted, manually, via sudo aptitude (I think you could use apt or synaptic or any other package manager as long as you have tight control of it) and made sure that only a minimal number of packages/dependencies were being automatically added. The updates are both minor updates and so should be compatible with everything on Jammy; this is a little risky I think, but for minor updates and small numbers of packages it seems relatively low risk.
Once the demanded libc6 (2.36-0ubuntu4) and libx11-xcb1 (1.8.1-2) packages and their dependencies had been installed, I quit out of aptitude.
I then reverted the change to sources.list by commenting out the Kinetic line; cleared apt using sudo apt autoremove; sudo apt autoclean; then ran apt for an update and full upgrade with sudo apt update; sudo apt full-upgrade.
Finally, the 3 Mozilla packages -- Firefox 108.0, Thunderbird 102.6 and a Thunderbird locale file -- installed.
Now that apt was fully up-to-date, I could run do-release-upgrade with:
do-release-upgrade -m desktop --allow-third-party
The install has now completed, and the system is working as expected.
Note: in the past, I've updated by changing the all the /etc/apt/sources.list entries and the source.list.d/ file entries from one release name to another and just running an apt-update followed by dist-upgrade (for which full-upgrade is a synonym). That might have worked here; what I did just seemed like a quick fix without me having to look at whether do-release-upgrade was doing anything fancy under the hood. HTH. YMMV.
