Fastly is a Content Delivery Network (CDN) which uses Varnish as a caching HTTP reverse proxy.
It is bundled in the Magento Commerce Cloud platform.
Blackfire must be able to trigger PHP on the server where you are profiling
(see how the Blackfire stack works).
To do you must make sure that caches are bypassed the Blackfire HTTP headers are preserved:
X-Blackfire-Query, X-Blackfire-Response and X-Blackfire-Error.
Several options are available:
If you are a Magento Commerce Cloud customer, there's nothing you need to do. The Fastly bypass for Blackfire is a standard configuration
If you are a Magento 2 "On Prem" (EE or CE) user, please make sure that you use the Module for integrating Fastly CDN with Magento 2 installations. There is a specific setting that will activate the bypass in a single click.
Else, please make sure to refer to our doc on Bypassing Reverse Proxy, Cache, and Content Delivery Networks.