My guess here is the newer version profile has information that the older version can't read so it can transfer those information and data to a older version as that can not read the newer data. It has to be 1:1 matching Firefox version for the data to transfer over if your doing just "copy". So you need to do the proper Firefox FAQ data transfer methods just copying will not work.

A simple method is to start Firefox with the ProfileManager and create a new Profile with the name you want. Shut down Firefox then find the new Profile you created and delete everything inside the newly created Profile. Then copy and paste over the contents from the old Profile into the new Profile. Then start Firefox with the ProfileManager and it should be there as a Profile to use.

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thanks for the speedy replies. I'll try both approaches tomorrow and see what happens.

James, unfortunately, your suggestion didn't work. I created a new profile and copied the contents of the desktop one into it, but Firefox wouldn't run. I suspect, as markwarner22 said, there's something in the 122.0.1 profile that clashes with the 97.0.1 version of the browser.

I suspected that the problem might be that two different versions of Firefox were involved: 97.0.1 on the laptop, 122.0.1 on the desktop. I thought about updating the laptop one to 122.0.1 but in the end decided to go for the latest incarnation.

Even with 133.0 in place, simply adding the desktop profile to the relevant directory still didn't work: it wasn't recognised. But James' idea was successful: I copied everything from the PC profile into a newly-created one, and that was not only picked up, when set as the default it correctly produced the revised layout that I was used to.

With one exception: for some reason, the colour coding didn't work. My carefully chosen contrasting colour scheme wasn't there. I'll look into that today.

This new profile was developed some time ago with the help of a long and very detailed Q&A thread which I thought was in these forums. But curiously I can't now find it. Is there another area of the web where Firefox coding matters are discussed?

Thanks again to James for getting me this far.