Such settings are stored in prefs.js. Best is not to copy this file to a new profile since it stores internally used meta data that is profile specific, but only copy user_pref() lines you need to prefs.js in the new profile with Firefox closed.

If you created custom devices, I think they were added to a storage database at:

[profile.folder]\storage\permanent\indexeddb+++fx-devtools

You could try migrating that folder to your new profile.

Dear cor-el and jscher200, thanks for your answers.

As I was especially interested in copying my custom device settings, I found a line in the prefs.js file that starts with: user_pref("devtools.responsive.html.displayedDeviceList". But that contains only the names of my added devices. So copying only that line did not work.

I expect that jscher200 is right that the actual values for my custom devices are stored in the mentioned database. But I did not try to copy that folder, because I'm not sure what else I would copy to my new profile. The reason that I created a new profile was to get rid of all the "junk" that might have been collected over the years in my old profile.

So in the end I just manually created my custom devices again in the new profile; not so much effort for only 4 devices.