Did you check if the Safari HTML backup contains all the Feed URLs?

You can open this HTML file in a Firefox tab for inspection.

I don't understand how this relates to my question.

Clarification: previous post:

I apologize if the previous post appeared rude rather than simply concise; that was not my intent. Rather, I'm assuming there's something additional that I need to do, and therefore trouble-shooting at this point I believe is premature.

My feeds (all in the format indicated in the original post above) — 507, as I initiated Open All in Tabs to get the exact count — are in a folder in my "Bookmarks Toolbar", exactly as they were in Safari (though I had used a tabbed folder in Safari).

In Safari (on 10.6.8, with its integrated RSS reader — that Apple replaced in 10.10 with a far more rudimentary version insufficient for my needs, and hence Firefox is now my default browser, most likely forever) clicking that tabbed folder in my toolbar generated a page with all new posts from my 507 RSS feeds (i.e. generated by the integrated RSS reader).

I didn't expect Firefox to operate the same, as my understanding is that I need to do something further with that folder of 507 feeds (hopefully not one at a time, though it's definitely time to dial back my reading  ;-) ) to make them into what Firefox refers to as "Live Bookmarks."

I just checked to insure I am correct to this point, by attempting to "Open All in Tabs" and Firefox did what I expected: rather than activate them as "Live Bookmarks (i.e. the equivalent, I assume, of Safari launching its integrated RSS reader and loading the new posts from each) it did, in fact, attempt to load the actual RSS feed page of each of the 507.