Think I figure it out. Nothing I tried, script-wise, would filter or change them - not even changing the "ui-color". So I started looking at the log window and found it showing:
14:47:55:1667 Fiddler.Network.ProtocolViolation - [#184] The Server did not return properly formatted HTTP Headers. HTTP headers should be terminated with CRLFCRLF. These were terminated with LFLF.
for these sessions. So I found that by going to "Tools>Options>General" and setting protocol violations to "Do nothing", the "/ChromeExt/" warnings stopped showing up in the session pane and it is much cleaner.
So, these are warnings and not sessions showing in the pane.
Hope this helps - cheers!