The prefs.js file stores several lines related to your Sync setup. I think you need to remove these lines from the prefs.js on the second device before launching Firefox, but I have not tested that myself.

Note: to open/edit a .js file, do not double-click it -- that causes the file to execute as a system script. Instead, right-click and open with a good text editor that won't mess up the code with formatting. For example, I use Notepad++ (https://notepad-plus-plus.org/).

These are the lines that have the potential to cause the problem (I've obscured the data, of course):

user_pref("identity.fxaccounts.account.device.name", "Windows10");
user_pref("identity.fxaccounts.account.telemetry.sanitized_uid", "blahblah");
user_pref("identity.fxaccounts.commands.missed.last_fetch", 1657760225);
user_pref("identity.fxaccounts.lastSignedInUserHash", "blahblah");
user_pref("identity.fxaccounts.toolbar.accessed", true);

By the way, I suggest carrying over the entire Firefox folder, not just a single profile folder.

C:\Users\<username>\AppData\Roaming\Mozilla\Firefox

(If needed, set Windows to show hidden files and folders: https://support.microsoft.com/en-us/windows/show-hidden-files-0320fe58-0117-fd59-6851-9b7f9840fdb2 )

The Firefox folder contains two key index files (installs.ini and profiles.ini) that Firefox uses to locate your profile.

But if you forgot, it's not the end of the world, you can still train Firefox to use your cloned folder with a little extra work.

Thank you for your reply @jscher2000, those settings are good to know.

Based on your input i had a look and also noticed several `services.sync.*` preferences in `about:config` so i'll reset those back to default too in the cloned profile;

I will give it a try soon and report back :-)

I did get signed out on my old computer, and had to sign in again. So it seems there were still some references somewhere in other files / databases.

However after signing in again, everything seems to work fine :-)