The statement that you are having trouble with is much older than Einstein, and it is very nearly true. Firewood is mostly cellulose, with a little water. When you burn it, the water evaporates, and then (if it rises into cool air) condenses. The cellulose consists of hydrogen, oxygen, and carbon; combining with the oxygen of the air, they produce carbon dioxide and more water. Those materials float, but they have mass, and if you add up the masses, you will indeed find that they are still "50 pounds of something". Lavoisier, in the 18th century, showed that by burning things inside sealed jars; the jar & contents always weighed the same before & after.

Einstein's contribution was to say that the above is not *quite* true. The energy (light & heat) given off also have mass, and if you could weigh the jar very precisely after it had cooled down, you would find that it weighed a little less. The deficit is so tiny (on the order of a part per billion) that it cannot be detected directly, so "matter is conserved" is for practical purposes correct. In nuclear reactions the energies involved are much greater, and the corresponding mass is on the order of a percent and actually shows up as a difference in the measured masses before & after.