Firefox Health Report records some identifiers to keep track of clients and uploaded documents.

Identifier Types

Document/Upload IDs

A random UUID called the Document ID or Upload ID is generated when the FHR client creates or uploads a new document.

When clients generate a new Document ID, they persist this ID to disk before the upload attempt.

As part of the upload, the client sends all old Document IDs to the server and asks the server to delete them. In well-behaving clients, the server has a single record for each client with a randomly-changing Document ID.

Client IDs

A Client ID is an identifier that attempts to uniquely identify an individual FHR client. Please note the emphasis on attempts in that last sentence: Client IDs do not guarantee uniqueness.

The Client ID is generated when the client first runs or as needed.

The Client ID is transferred to the server as part of every upload. The server is thus able to affiliate multiple document uploads with a single Client ID.

Client ID Versions

The semantics for how a Client ID is generated are versioned.

Version 1
The Client ID is a randomly-generated UUID.

History of Identifiers

In the beginning, there were just Document IDs. The thinking was clients would clean up after themselves and leave at most 1 active document on the server.

Unfortunately, this did not work out. Using brute force analysis to deduplicate records on the server, a number of interesting patterns emerged.

Clients would upload a new payload while not deleting the old payload.
Divergent records
Records would share data up to a certain date and then the data would almost completely diverge. This appears to be indicative of profile copying.
Records would share data up to a certain date. Each record in this set would contain data for a day or two but no extra data. This could be explained by filesystem rollback on the client.

A significant percentage of the records on the server belonged to misbehaving clients. Identifying these records was extremely resource intensive and error-prone. These records were undermining the ability to use Firefox Health Report data.

Thus, the Client ID was born. The intent of the Client ID was to uniquely identify clients so the extreme effort required and the questionable reliability of deduplicating server data would become problems of the past.

The Client ID was originally a randomly-generated UUID (version 1). This allowed detection of orphaning and rollback. However, these version 1 Client IDs were still susceptible to use on multiple profiles and machines if the profile was copied.