The threat model is reasonable behavior + a single warrant or data leak. It may not be the most likely way of compromise, but it's very possible, and such small insecurities add up. Mozilla should be making such compromise less likely, not more.

How do they add up? Having your IP address 10 times won’t make anyone ever of your medical history. It is not cumulative.

They add up in the sense that each increases the likelihood you will be compromised, and reduces the set of activities that you may safely and privately perform.