Do not put an airtight lid on the jar. As the seeds start to ferment they will produce a gas which may cause the jar to explode if the gas can't escape!
Instead cover with a piece of kitchen towel secured with an elastic band. This will allow the gas to escape whilst keeping the fermentation smell in and keeping flies out.
If you are saving more than one variety of tomato seed attach a label with the variety name onto the jar. One tomato seed looks very much like another and you might not remember which is which otherwise.