Are you posting or sending an e-mail?

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

I'm not emailing; I'm using the buttons at the bottom of the post: Reply All, Reply to Author, Forward. The Reply to Author is greyed out. I started Firefox in Safe Mode and the problem is still there. (By the way, how do I know whether a session of Firefox is in Safe Mode? I don't see any obvious messages that tell me that.)

Just to be thorough: I am starting Safe Mode by holding down Shift while I click the Firefox icon. I didn't see a functional dialog box in the link you provided.

I realized that I had stumbled into the new Google group experience, and decided to revert back to the old experience -- the problem is *not* there. It is also not there in the Safe Mode version. (When I reverted to the old experience, all of my Firefox sessions, whether in Safe Mode or not, seem to know that I've reverted.)

Should I just stay with the old experience, and try the new experience some time in the future? I'd rather keep up with the Joneses on this one, but I don't want to spend more time on a relatively trivial issue. Thanks.

. . . . and decided to revert back to the old experience

What do you mean by this?

When I am in the new Google groups experience there is a button that allows me to go back to the old Google groups experience (basically the way the user interface works). I decided to go back to that mode to see if the problem was still there. When I clicked the button, I was back in the old mode, and the 'Reply to Author' option was not greyed out; I was able to use it.

I would definitely report this issue to the folks at Google Groups (I assume they have a support group) so they can work on a solution. It may be a bug in the new version.