two issues with modern windows:

side jambs have small (loose) foam blocks which seem marginally useful in keeping the air from coming through the rails where the windows slide -- what to replace with?

they have a built in weatherstripping (fuzzy kind) on the side, but in a couple of cases that does not fully reach the side frame, leaving 1/8th gap. trying to replace with foam tape was useless as it peels off when reclosing windows.