proj4() expects a projection definition, not just an ellipsoid.
From http://proj4js.org/ you might read that
WGS84 is the only named projection defined in proj4js. You have to use the listed EPSG codes or define the projection in full.
So your command might be:
var firstProjection = "+proj=longlat +ellps=GRS80 +no_defs";
var secondProjection = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs";
Apart from that, your coordinates are not in the range of proper WGS84, that is +/- 180/90 degrees. From the comments, it seems you are looking for
var firstProjection = "+proj=tmerc +lat_0=31.73439361111111 +lon_0=35.20451694444445 +k=1.0000067 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +towgs84=-48,55,52,0,0,0,0 +units=m +no_defs"
var secondProjection = "+proj=longlat +ellps=WGS84 +datum=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs";
Note that proj4js does not have all EPSG code numbers stored internally, like Proj.4 and GDAL have.