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.

Note that you can register a profile in the Profile Manager by using the "Choose Folder" button. See "Start the Profile Manager when Firefox is closed":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles
• https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

Some files have absolute paths, so copying a full profile folder content to a different profile is not recommended.

cor-el, thanks for that; it might be the reason why I'm getting only partial success. I'll investigate the option to register a profile.

An update: I have also been running Firefox 122.0.1 on a spare backup laptop: on this setup, my reconfiguration profile works perfectly with layout tweaks and new colours all intact.

As an experiment, I updated Firefox to the latest version. As with my main laptop (as reported above) the profile wasn't recognised and I was prompted to create a new one. As before, I did so and copied into it the entire contents of my preferred profile. A restarted Firefox picked up this profile and I was able to set it as the default, but once again though the revised layout was more or less intact, there were a few anomalies and the colours weren't picked up at all.

So it seems that there are two possibilities: either the profile isn't compatible with the latest software, or it's my copy-and-paste of the old profile into a new one which - as cor-el suggested - is mucking everything up. I must see if I can find out why simply putting my restyling-profile as a whole into the correct directory isn't getting it recognised.

Thanks to everyone for your patience and thoughts so far.

Did you update via "Help -> About Firefox" to get to the current version or did you install the latest version as there are so called watershed versions that have migration code to update files to newer schemes?

Thanks for the speedy reply. I used Help - About Firefox. Would downloading the latest version from the Mozilla website be a better approach?

No, best is always to use "Help -> About Firefox" like you did.

You can copy certain files with Firefox closed from one profile folder to another profile folder to transfer your personal data. Note that best is to avoid restoring a full profile folder and only restore important files that are safe to restore/transfer.

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (encrypted logins) and key4.db (encryption key/primary password) for logins saved in the Password Manager
• cert9.db for certificates stored in the Certificate Manager
• persdict.dat for words added to the spell checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

Again, thanks. If those are the only files it's considered safe to restore, is there a way in which can I continue to use my extensively-coded chrome folder?

You can copy the chrome folder as well, but if the profile had been used by an earlier Firefox version then you may need an update for CSS code in userChrome.css. Just be cautious with files like prefs.js and indexedDB (storage folder) and installed extensions.

That's very helpful, thanks. Do new Firefox versions usually make it necessary to update CSS coding? If so, are any required changes documented anywhere?

Do new Firefox versions usually make it necessary to update CSS coding?

A few times a year, there is a change in Firefox major enough to break userChrome.css rules.

If so, are any required changes documented anywhere?

Nothing official. The r/FirefoxCSS subreddit will have threads for major issues, and forum volunteers here might remember some of the fixes if they were for recent versions of Firefox.