To follow up on Alexandre’s answer, note that “Maßen” is pronounced with a long “a” before the “ß,” while “Massen” has a short “a” before the “ss” — that is the purpose of the new spelling reform.

Likewise, “dass” has a short “a,” so the new spelling rules specify that it is no longer to be spelled “daß.”  Of course, people in their late 30s or older will probably continue with the centuries’ old tradition of always writing “ss” as “ß” in syllable-final position. (I originally wrote “late 30’s” with an apostrophe, but then remembered that the long-standing tradition of using an apostrophe to pluralize numbers was dropped a few decades ago on the grounds that it’s not a possessive. Ah, well….)