Extensions may not work properly when changing the profile location until you delete extensions.databaseSchema in about:config and then restart the browser.

Actually, the answer didn't really answer my question. Apart from the specific suggestion about extensions, are there any other points/steps I have missed

Sync is your answer. see screenshot https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer https://support.mozilla.org/en-US/kb/how-do-i-choose-what-information-sync-firefox

I have 1 Mozilla account on different devices and versions of Firefox.

What I want to do is transfer my profile from the Win7 machine to the Win10 one, and then close down the Win7 one, as it is slowly but surely falling over. So I need all the info to be permanently held on the Win10 one - pref before the Win7 system falls over.

That's what it will do... You see the options to check on both computers and once you log into the Win10 machine with the same account, the data will just populate.

That's why I showed the screenshot of my other laptops that are synced with the same data.

So presumably I need to add the other copy of Firefox to my Mozilla account?

I've only just installed it - haven't personalised it yet - or got email on there either.

BTW - wrong forum, was going to ask if it would work with T'bird. But just checked and it won't. So back to plan A. (My T'bird profile was 8Gb last time I backed it up!)

PS - I can't see an option to add in another copy of Firefox to my account (just looked). Prob is that Win10 has assigned me an aribtrary username. Do I just go to Mozilla support from the other copy and sign in under the username here?

I don't do T'bird. Not a fan of local email apps. Are you logged into sync on the Win 7 computer and have the sync options shown in my screenshot checked?

On the win10 computer, sign into sync with the same account that's on the win7 computer, make sure the sync options are checked, and watch the magic.

That's it.

I'll mark this as solved - 'cos I think I see how this works. In the immortal words of a certain former State Governor "I'll be back" if I'm wrong/can't get it to work

We'll leave the lights on for ya.  ;-))