What could happen is that Search for text when you start typing is activated in the Options > Advanced section > General tab, so it starts searching for text. Disable that option to see if it stops. However, this should only happen in non-text fields, so the real question is what triggers it. Or perhaps you just need enter a text field first with a mouse click? If so, it is expected behavior.

If not, can you reproduce the issue when starting Firefox in Safe Mode when the option is enabled? If so, check and disable or update any faulty add-ons - see the article for more info.

Turning off the searching for text means I can't enter any text in the reply box. How do I start Firefox in Safe Mode without disabling the add-ons? Which article?

You don’t turn off the searching for text - enabling the option makes a find bar pop up as soon as you type, but outside a text (editable) field. Normal search is different, still possible and can be performed by the Ctrl-F shortcut, menu button or Edit menu. Does the same happen when the option is disabled?

Also see this article.

Disabling add-ons is the point of Safe Mode to quickly see if one causes the issue, if there is one. Or you can disable them one by one in regular mode to see when the issue stops, but Safe Mode does more. See the article referred to by the Safe Mode link.