Just in case anybody else is experiencing strange forum behavior when using Firefox like I do:
With old Firefox browser 52 on Ubuntu it behaves abnormally when replying to an existing thread - the cursor may disappear in the message typing area. With Ubuntu 12 I’m running I cannot update Firefox the usual way.
Even Firefox 58 on Android behaves strangely, even a search cannot be typed. Just updated to Firefox 67 for Android before creating this post - it seems better.