I checked that page with the new Firefox immediately after setting up the preferences, and before doing anything else to it. Half the page disappeared again.
Turns out it was the type size. I had set the minimum to 18. As soon as I lowered it to 13, the missing half of the page appeared. That's why the Terminal safe start made no difference and why SeaMonkey couldn't read the page. Live and learn. (But all the browsers that could read the page have the same minimum type size. Not having Gecko engines and a slightly different screen resolution in 9's Mozilla make the difference, I guess.)
I put the old Firefox back, along with the Profile folder and the plist. Reloading 20 extensions, going through the user.js file line by line and changing a bunch of AppleScript pathways because of the new randomly generated name given to the slush-pile folder aren't much fun.
If nothing else, the Mighty Mouse taught me something new.