From the article, nothing alludes to rate-limiting:
For example, we may log whether a connection succeeded or failed, or record that your account used 2 GB of data on a certain day.
"This helps us improve performance, prevent abuse, and plan for growth.
No this is incorrect
This helps us improve performance, prevent abuse, and plan for growth. Logs linked to your account are automatically deleted after 3 months. Importantly, Firefox VPN never logs the websites you visit or the content of your communications.