How to allow (enforce) page-breaks for (inside) <iframe> elements?

Question

Occasionally I want to print web pages which contain embedded long (high) <iframe> elements (into a pdf file).

Unfortunately these <iframe> elements never get cut when it is necessary. Instead the top of <iframe> content is shifted to the next page.

All other stuff (like images or tables) are successfully cut (when necessary) but not <iframes>.

I tried a lot of CSS rules like

page-break-inside: auto !important; or break-inside: auto !important;

....but without any effect.

Is this a special problem for Firefox (on windows 10)?

How can I really, really enforce page-breaks even for <iframes>?

see above for description