Judging from the height of your toolbar area, I'm guessing you have:

• Title bar: customized "on" (showing tab title text)
• Menu bar: customized "on"
• Tabs bar: separate rather than merged into the title bar
• Main toolbar
• Bookmarks toolbar

I suspect this configuration doesn't get as much testing, but still, I don't see why the title bar should suddenly lose its theming.

By the way, when this happens, does it help to go to full screen (F11) and then out of full screen (F11 again)?

Thanks for replying @jscher2000. Yes - the title bar immediately appears on full screen, and remains visible returning.

This isn't a big deal, but is frustrating with extra steps for simple habitual navigation. I have work-arounds to force it to appear, but would be great to not have-to, just to perform very common and repetitive tasks. Thanks!

Are there any settings for the page display in about:config or anything? Or, ideas where the issue could be so I can chase it? My themes have worked for years just fine. Still frustrated. :(

When you say page display, I think of the Website Appearance setting (Change website appearance settings in Firefox). For the toolbar area, I think themes are the only built-in tool. If there is a problem where Firefox seems to forget to override the standard Windows title bar, I doubt there is a setting related to that.

Possibly related: my system was recently having problems. At one point, when Firefox became nonresponsive, the top bar had throwback Windows 7 theming that I never see anywhere else. So strange.