Conclusions obtained through PS screenshots
When setting td{height:22px;}, the page displays: ie is 22px, ff is 21px; Practice has proved that ie will always be 2px larger, and ff will be 1px smaller.
When setting td{line-height:22px}, the page displays: ie is 24px; ff is also 24px; 2px larger than the actual one, but consistent.
You can also test it yourself.