Why anti ETag fingerprinting browser extensions won't work if I block images using other addons, like uBlock or uMatrix?

I’m trying to use one of these Firefox extensions suggested by ghacks:

• Header Editor;
• ETag Stoppa.

As described by the wiki, this is what they do:

Allows you to run Rules to modify headers such as blocking ETags

The problem is: these addons won’t work if I block images in uMatrix, at all. Hence, I’d like to know why is that.

Also: do these addons work by preventing first-party image eTAGs or third-party as well?

expected result

• install ETag Stoppa;
• disable images globally for all parties (uMatrix);
• visit Cookieless cookies;
• store message;
• clear cache;
• re-visit Cookieless cookies;
• don’t see any previously storaged message.

what’s actually happening

• install ETag Stoppa;
• enable images globally for all parties (uMatrix);
• visit Cookieless cookies;
• store message;
• clear cache;
• re-visit Cookieless cookies;
• see previously storaged message.

disclaimer

1. I’m cleaning browser cache by reopening it because I have prefs for it.

links

• Cookieless cookies by lucb1e