I found the answer myself, or should I say, an answer. The answer is: add the iframe via javascript after the page loads (or near the end).

Reference: Javascript: add iframe element after page has loaded - thank you Stackoverflow community.

Specifically, instead of

<iframe src=somepage.html>

I use

const iframe = document.createElement("iframe");
iframe.id = "framex";
document.getElementById("whereIwantIt").appendChild(iframe);

That prevents Firefox from preloading the html of the iframe until I ask it to, later, by giving it the source file.