Yes, it was switched to "active tab per window" only a long time ago to speed up making the browser available. if you are only restoring a couple dozen tabs, here's how to have Firefox load them all at once. Note that Firefox may load from cache if the cached files have not expired, so you still might need to reload a page to get the freshest content.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste on_demand and pause while the list is filtered -- Firefox should display two preferences, one for regular tabs and one for pinned tabs

(3) Double-click the browser.sessionstore.restore_on_demand preference to switch the value from true to false

Oh, on the cached files point, you could set Firefox to clear the cache when it closes, and then it will have to re-retrieve the pages from the servers when they are restored. Maybe you were doing that before?

Hhmm, I might be a little confused.

"Note that Firefox may load from cache if the cached files have not expired, so you still might need to reload a page to get the freshest content."

To be honest, I don't know how to find out when a cached page expires. Are you saying that even if I make the edit you gave me, I'll still probably need to manually refresh most pages (unless I set it to clear the cache every day)?

Yes, the preference controls whether the tab is loaded on activation versus automatically at startup. It doesn't set Firefox to ignore cached files.

Ok, I think I have all the info I need.

You answered my question, even though I'm not sure if I will use it in the long term. But I'll try it out for a while, and mark the question solved, for others who may search it out.

Thank you very much!!