Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, check either:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

If you closed out of Firefox using Quit with your windows still open, then those open windows should be restored automatically without having to use that menu. However, if you closed individual windows before quitting, then you need to use that menu.

Anything there?

Are you sure the window belongs to the separate profile? Visit about:profiles in the address bar and look for "This is the profile in use and it cannot be deleted". Give each profile a different theme to avoid confusion. It may be easier to use a separate installation like the ESR version (115) or Developer Edition (early beta).

@jscher2000

    "3-bar" menu button > History > Restore Previous Session wasn't available
   (menu bar) History > Restore Previous Session works but is awkward.

@zeroknight I've installed a separate ESR version with a different theme, and named it "Firefox WORK". This seems to work.