Somebody shared this in a different comment, but the OneTab extension looks like exactly what you are asking for: https://www.one-tab.com/ (minus it being builtin).

I actually used to use AppleScript to grab open tab URLs in Chrome but I switched to Firefox and AFAIK, Firefox doesn't support AppleScript.

    tell application "Chromium"
        set tabCount to get count every tab of window 1
        repeat with i from 1 to tabCount
            set currentUrl to get URL of tab i of window 1
        end repeat
    end tell

I saw mention of this on HN somewhere previously, but I've found that Tablist[0] is simple (a couple dozen lines of code) and works well.

[0] https://github.com/slymax/tablist (links to Chrome and Firefox extensions there)