calc max-width for Firefox

account for prompt width

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

@@ -107,7 +107,7 b' div.output_subarea {'
     padding: @code_padding;
     .box-flex1();
     // appears to be needed for max-width in children to mean anything on Firefox:
-    max-width: 100%;
+    max-width: calc(~"100% - 14ex");
 }
 
 /* The rest of the output_* classes are for special styling of the different

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

@@ -890,7 +890,7 b' div.output_subarea {'
   box-flex: 1;
   /* Modern browsers */
   flex: 1;
-  max-width: 100%;
+  max-width: calc(100% - 14ex);
 }
 /* The rest of the output_* classes are for special styling of the different
    output types */

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

@@ -9682,7 +9682,7 b' div.output_subarea {'
   box-flex: 1;
   /* Modern browsers */
   flex: 1;
-  max-width: 100%;
+  max-width: calc(100% - 14ex);
 }
 /* The rest of the output_* classes are for special styling of the different
    output types */