Have no issue with the link provided and the comments are expanded already. Did drag copy and pasted to notepad without issue.

Congrats! What version of Firefox and Linux were you using to get this result? If you're using Linux, were you using xfce?

I don't have ESR, so I can't replicate that.

Is this problem limited to the dynamically inserted comments? When I look at the page inspector (attached), they look identical in structure to the static comments, so it's hard to understand why they would be uncopyable if the others are copyable.

Apparently this is an issue with the mozilla-firefox-60.6.1esr-x86_64-1 version, provided with Slackware current updates. Removing this version and installing to mozilla-firefox-66.0.2-x86_64-1 resolved the issue. My take-away with this is that distro term 'current' is not what was expected. Apparently 'current' is what the maintainers currently have available. Your good perception in noting that your version was not an 'esr' triggered the path to resolving the issue. Thanks. Many times the responses are simply 'it works for me' which do not help very much at all. Just a little more info can make a world of difference.

Thanks for the update.

Customized distributions may also have extra extensions and settings tweaks, so it could be difficult to track down exactly what was causing the problem.

Agreed. Resolving these issues can take qutie a bit of effort, especially without intimate knowledge of the nuances of that version. This issue could have been how the esr version handled style sheets, but like so many guesses, this could have led to many hours of 'learning' only to find out it is something else. I'll hit the 'easy' button this time.