[Discover] Fix header row of data grid in Firefox (#101374)

* [Discover] add fix of row header in firefox from previous version

* [Discover] add link comment to the issue

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

src/plugins/discover/public/application/components/discover_grid/discover_grid.scss

@@ -47,6 +47,9 @@
 
 // We only truncate if the cell is not a control column.
 .euiDataGridHeader {
+  // This display property is temporary until https://github.com/elastic/eui/issues/4729 is resolved.
+  display: flex;
+
   .euiDataGridHeaderCell__content {
     @include euiTextTruncate;
     overflow: hidden;