Using Firefox 55, I tried with uMatrix 1.0.0 and 1.0.1rc1
However the option "Collapse placeholder of blocked elements" appears to be ignored. Regardless if you check or uncheck it, the placeholders of blocked elements are not collapsed, so you are left with blank placeholders littering the page.
Note that uBlock Origin has the same option, but uBlock Origin correctly collapses the blocked elements. Example page http://reddit.com