The profile folder you copied should be from the Roaming data folder on your old computer and it will go into the Roaming data folder on your new computer.

You can't really do a direct substitution because the folder name will be different and it can't just be changed.

Instead, you need to paste the folder there and keep the name the same as it was from the original computer. Then, using the Firefox Profile Manager, you need to create a new profile. At the screen where you are naming the profile, there's an option to choose an existing folder. Use that to select the profile that you copied over.

Hope this helps.

To avoid issues, best is to only transfer specific data files you need and not copy a full profile. You would only need files from the location for the main profile in "AppData\Roaming". The other profile location in "AppData\Local" stores temporary files like the disk cache an startup cache.

You can copy specific files with Firefox closed to the current profile folder to recover personal data.

• 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 (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling 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)

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Many thanks, Wesley and cor-el for your prompt replies. All I really need is the Bookmarls & History so I've copied the old places.sqlite file into the new profile folder (with Firefox not running!) but the bookmarks etc are not showing up. Is there anything else I need to do? I'm wondering if the fact that the Windows 10 machine is 64-bit whereas the old PC is of course 32-bit could be a problem? Thanks!

OK I think I've solved it - I went to the about:profiles page and that led me to the solution. It turns out the active profile is the one with the suffix .default-release - not the one with the suffix .default. I'm new to Windows 10 and it seems a lot more convoluted than Windows 7! Thanks again!

Note that I posted in my above reply to use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder (or use the about:profiles page). Using about:support is usually easiest to go to the profile folder as there is only one choice. The about:profiles page can reveal whether more profiles are registered and available as choices in case you lost data or need to recover data from another profile.