set max-width on output_subarea

for Firefox

IPython/html/static/notebook/less/outputarea.less

@@ -106,6 +106,8 @@ div.output_subarea {
     overflow-x: auto;
     padding: @code_padding;
     .box-flex1();
+    // appears to be needed for max-width in children to mean anything on Firefox:
+    max-width: 100%;
 }
 
 /* The rest of the output_* classes are for special styling of the different

IPython/html/static/style/ipython.min.css

@@ -890,6 +890,7 @@ div.output_subarea {
   box-flex: 1;
   /* Modern browsers */
   flex: 1;
+  max-width: 100%;
 }
 /* The rest of the output_* classes are for special styling of the different
    output types */

IPython/html/static/style/style.min.css

@@ -9682,6 +9682,7 @@ div.output_subarea {
   box-flex: 1;
   /* Modern browsers */
   flex: 1;
+  max-width: 100%;
 }
 /* The rest of the output_* classes are for special styling of the different
    output types */