+++ This bug was initially created as a clone of Bug #1630939 +++
Splitting this discussion out of bug 1626935.
It seems the skip list service gets created relatively early during startup (AFAICT through https://searchfox.org/mozilla-central/rev/567b68b8ff4b6d607ba34a6f1926873d21a7b4d7/netwerk/url-classifier/AsyncUrlChannelClassifier.cpp#792 as soon as the first http channel is
I'm seeing the url-classifier run and create features for requests from about:newtab (notably image requests from the snippets CDN), and I don't see any obvious reason we wouldn't run the same code for internal
fetch() requests, like the ones we use for update checks, captive portal checks, etc. That seems like it shouldn't ever result in anything getting blocked...
It also seems like we should avoid running them for document requests with system principal triggering principal (ie when the user explicitly requests opening a page in the toplevel docshell).
It seems like we could scope down when we need this information and thereby delay when we initialize this code, and that'd potentially help startup performance in the common case (without regressing things when automatic sessionrestore is enabled and we load webpages as the first thing in the initial window).