Hmm, that's a little tricky because any new profile you sign in to your Firefox Account will usually inherit all the information in the account.

Of course, you can create a new profile and NOT sign it in to your account, but if you want to sign into your Firefox Account in a new profile, you may need to first flush all the data in your account. The only method I know of to do that is a password reset (the kind of reset you do if you do not know your password). More info in the following article:

I've lost my Firefox Sync account information - What to do

But I don't know whether that might have any unwanted side effects for other services (Mozilla VPN, Relay, Monitor, etc.), so you wait and see whether you get some better options.

If you are connect to Sync and then delete all data in Firefox that is Synced then this should propagate and clear data on the server as well.

You can install the About Sync extension in Firefox 72 and newer on desktop to check what data is stored on the Sync server via the about:sync page provided by this extension.

• About Sync https://addons.mozilla.org/firefox/addon/about-sync/

Close and restart Firefox to access the about:sync page added by this extension (see the about:about page for possible about pages).