Sorry, my Internet cut out right before I could post this:
Note that I’m using desktop Safari on Mac. It looks like on Chrome the numbers aren’t selectable. So, for Chrome, linenos=inline is indeed a good workaround, but it doesn’t work for Safari on Mac or any browser on iOS/iPadOS.
I marked my CSS workaround as the post solution for now, since it’s currently the only way to get correct out-of-the-box behavior for Hugo and Bootstrap. Once the Chroma issue is fixed, then setting markup.highlight.lineNumbersInTable to false would be the best solution, in my opinion.
For what it’s worth, in my opinion, if that Firefox bug hasn’t been fixed in seven years, then it’ll never be fixed. Just write it off and make the change anyway. Firefox users can complain to Mozilla to fix their stuff, or switch to a “better” browser.