moz-extension: URLs being passed into TorDomainIsolator

Noticed something strange that occurs in my dev build and 15.0a2, that didn't occur in 14.5.

1. Set browser.tordomainisolator.loglevel set to All.
2. Bootstrap and visit example.org.

In the console I see

TorDomainIsolator: Requested https://example.org/ via example.org:0:d8ff882fa3071b07b564cef3ba7b61be79d76df82275cc13207bccc38da1e6ba 
TorDomainIsolator: Updating circuit example.org:0|d8ff882fa3071b07b564cef3ba7b61be79d76df82275cc13207bccc38da1e6ba 
TorDomainIsolator: Missing data to associate to a browser 
TorDomainIsolator: Requested https://[ff00::]/moz-extension://37e71612-33b3-44df-a77b-c073912c9028/nscl/common/SyncMessage/request.json?id=c6f3ee5e-ff5a-4efd-827a-d9ba2ac36d13%3A99fc3ed9-150a-46ee-af30-5d8fe31626b7 via example.org:0:d8ff882fa3071b07b564cef3ba7b61be79d76df82275cc13207bccc38da1e6ba
TorDomainIsolator: Matched browser with browserId 2
TorDomainIsolator: Requested https://example.org/favicon.ico via example.org:0:d8ff882fa3071b07b564cef3ba7b61be79d76df82275cc13207bccc38da1e6ba

In particular, there is a request to https://[ff00::]/moz-extension://37e71612-33b3-44df-a77b-c073912c9028 (I think this is NoScript) via example.org. I.e. a request is being made to moz-extension via the example.org principal.

What changed in 15.0 that the request is now going through the proxy logic?

/cc @ma1 @pierov