I use Firefox with Google Sheets. When filling in the cell and pressing enter Google sheets automatically adds a new line. It only seems to happen then you press enter. It doesn’t happen when you press tab or click out of the call. It also only happens in Firefox and not Google Chrome. Being cynical, I wonder if Google know about this bug and aren’t fixing it so that people migrate from Firefox to Chrome. Anyhow, it is quite simple to fix it.
Just go to Tools. You will find that autocomplete is enabled.
Disable autocomplete and the problem will disappear,
Hope this helps someone.