For me on Linux with a light theme, I see a blue outline. It is possible that with your dark theme red is used because blue wouldn't give enough contrast, so you can try a light theme.

thanks for the reply but this is not a cosmetic issue. All 4 browsers on my other desktops have a standard blue outline as the browser in question did also until after the firefox update. As mentioned the red outline showed up after the last Firefox update. I am perceiving this as an issue either with the update or some type of warning.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems