This should help you Disable Firefox Sync on a lost phone or tablet

OK. I've read the article. If I understand it right, you can prevent future synchronization on a lost device but not remove already synchronized information from said device?

Options in Sync online (https://account.services.mozilla.com/) like "Clear your Sync data" or "Delete your account" do not do the job? (In fact, I'm not sure what theses options do or do not do.)

Thanks

The only way to prevent another (lost) device from accessing the Sync account is to change the password for the account and possibly generate a new Sync key.
You first need to unlink all other devices and setup Sync again on all devices once you have changed the password.

• https://support.mozilla.org/kb/ive-lost-my-firefox-sync-account-information

On the page you linked it is mentionned (under Generate a new Recovery Key):

"Warning: This will erase all of your data stored on the server. New data will be secured by this key and then uploaded. Your other devices will stop synchronizing, unless you update them with the new Recovery Key."

Does this means all information will also be lost on the devices or does it merely mean that what has been synchornized will stay unchange on all devices but no more information will be sync from Mozilla servers?

I want to know if there's a way to delete the information on the devices losts/stolen and not merely prevent access to the Sync account.

I realized this question betrays my lack of understanding about how Sync works, but hopefully it will be useful to someone else.

Thanks

Data stored on a specific device isn't effected .
Only the synchronization between the devices is temporarily lost until you have set up and connected all devices using the new password and sync key.
But it is best to first unlink all devices to make sure that no syncing is interrupted.