how to read and stop content using firefox webextension/electrolisys add-on

I wrote a functioning internet content filter firefox addon that uses XPCOM methodology described here to intercept, analyze, and cancel incoming web content which met certain criteria.
However, XPCOM is being replaced by webextension/electrolisys and I cannot find an API that will do the same thing. I checked webNavigation and webRequest APIs but could not find the capability I need.
How can I intercept, analyze, and cancel incoming web content using mozilla webextension/electrolisys methodology in an add-on?
Thanks in advance.