+++ This bug was initially created as a clone of Bug #1266889 +++ We don't want to load the plugin when we're going to block the SWF anyways. We need to check the block list before instantiating the plugin. The plugin instantiation fix in bug 1266889 is not e10s-compatible, but that is OK for Beta 47 when e10s is not enabled. We need an e10s-compatible fix for Aurora 48+, where e10s is enabled by default.
Tobias said he will add an end-to-end test to verify that plugin instantiation blocking actually works, instead of just testing channel classification.