Whether Ember Weaver has first strike matters only at the point that the normal combat damage step would begin, and only if there hasn't already been a first-strike combat damage step that turn. If any of your creatures has first strike at that point, a first-strike combat damage step is created. -- If Ember Weaver has first strike at that point, it will assign and deal combat damage during that step (even if it loses first strike before that damage is dealt) and it won't deal combat damage during the normal combat damage step. -- If Ember Weaver doesn't have first strike at that point, it will assign and deal its combat damage during the normal combat damage step (even if it gains first strike before then).