Firefox has two different ways to remember zoom: "by tab" or "by site". A long time ago, Firefox switched from "by tab" to "by site", but perhaps yours got switched back? Could you check:

The about:config method should work for about:newtab, but extensions can override it (and a user.js file can revert it at the next startup).

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste site and pause while the list is filtered

(3) If the browser.zoom.siteSpecific preference is user set to false, double-click it to restore the default value of true.

Does that explain it?

Is site specific zoom enabled?

• http://kb.mozillazine.org/Zoom_text_of_web_pages
• http://kb.mozillazine.org/browser.zoom.full
• http://kb.mozillazine.org/browser.zoom.siteSpecific

• http://kb.mozillazine.org/toolkit.zoomManager.zoomValues

• http://kb.mozillazine.org/zoom.minPercent
• http://kb.mozillazine.org/zoom.maxPercent

browser.zoom.siteSpecific preference is set to true

site specific zoom is enabled, and works when I open a zoomed site in a new window, but it does not work when I open a zoomed site in a new tab.

If it does work in a new window then stay with that window and close the other, not working, window.

Oddly enough, that worked.

What I meant was, the new WINDOW was zoomed properly, but when I checked to see whether tabs would zoom properly from that new window, they did.

So, thank you. I still have no idea what went wrong, and I didn't actually do anything to fix it, but it is behaving for the moment, so I'm happy.