Did you compare the URL that Firefox tries to open with the URL that works in other browsers to see if there is a difference?

The URL is the same. What does that signify?

(NB: I forgot to note that the same problem occurs when I do this in a private window in Firefox.)

Do you have to authenticate to be able to access the page (i.e. a session ID cookie needs to be send) ?

You can check for issues with Total Cookie Protection.

• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/introducing-total-cookie-protection-standard-mode

Messing around (so to speak) with the Wordpress help folks, we were able to fix the problem. We did this by exercising the WP function for creating a new post from an existing one. Cookie-wise, the new post starts with a clean slate. Our imperfect diagnosis: the introduction of Firefox's 'cookie protection' features somehow corrupted the underlying cookies that the Wordpress generate-preview function uses, and no fiddling with the settings for the 'cookie protection' features was able to fix the problem. Anyway, problem fixed, and thanks for your help.

Did you try disabling Tracking Protection for the site?

Yes, for the record, (that is, I tried disabling Tracking Protection for that site), but it didn't fix the problem.