You said Mobile as the device but your posting said laptop - so which is which here? When one login to Firefox account AFAIK don't require a new profile unless one creates a new Firefox account.

I set up Firefox on a new laptop yesterday and it came up with all the bookmarks etc I had previously. I was prompted to set a profile so I set default user. Now I have lost everything I had. How do I get it all back.

You created a new Profile with the ProfileManager? https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles#

Hi Richard, were you prompted to create a new profile? That can happen if you launch an older version of Firefox than the last one you used on the same system, or if a critical file is locked.

To rule out the file lock issue, do a regular shutdown and restart of your laptop.

Then use the steps in the following article to test other profiles available to Firefox and see whether Firefox will let you switch:

Recover lost or missing Bookmarks

If Firefox won't switch because you have an older version of Firefox, then:

Update Firefox to the latest release

Can you get back with your data?

P.S. This forum doesn't accept email replies, so follow up at https://support.mozilla.org/questions/1419531

I followed jscher2000's steps and now have everything back. Thanks for your help.