Some websites just go horribly weird in Firefox, but whenever I search I cannot find anyone else with the same problem. It isn't fonts, it isn't character encoding.
If I press
Esc in that fraction of a second, then I get to see the real page.
That is often good enough, but I don't get any embedded content. In the above case, after trying the same URL in chrome and opera, I can see it is an embedded pdf.
So, embedded content is triggering some problem. It only happens with google sites (as in, websites that have been made from google documents - google search, gmail, etc. are all fine). I doubt it is needing a plugin, as chrome and opera work out of the box.
Firefox 55, 64-bit linux; but this has been happening a couple of years, so back to the Firefox 40s, at least, I imagine.