Bug 1872851 - Firefox View longer typed terms will extend the search …

…results box out of view in History section r=kcochrane,fxview-reviewers

Use zero width to indicate that the header should only take up the width of its parent container.

https://stackoverflow.com/a/55041133/6268626

Differential Revision: https://phabricator.services.mozilla.com/D198047

browser/components/firefoxview/card-container.css

@@ -123,6 +123,7 @@
 
 ::slotted([slot=header]) {
   flex: 1;
+  width: 0;
 }
 
 ::slotted([slot=secondary-header]) {