Bug 1820534 - Move front-end to modern flexbox. r=Gijs,dao,settings-reviewers,credential-management-reviewers,sgalich,devtools-reviewers,nchevobbe

Done mostly automatically via find/replace following the conversions
specified here:

  https://groups.google.com/a/mozilla.org/g/firefox-dev/c/9sGpF1TNbLk/m/QpU3oTUuAgAJ

For the most part I think the "flex: N N" usage could be simplified to
just "flex: N", but I wanted to preserve behavior (-moz-box-flex sets
both flex-grow and flex-shrink).

I changed legacy layout to also look at the order property rather than
-moz-box-ordinal-group because it made splitters and treecols easier (we
don't need to deal with both orders).

Differential Revision: https://phabricator.services.mozilla.com/D171715

browser/base/content/browser-siteProtections.js

@@ -235,8 +235,8 @@ class ProtectionCategory {
     }
 
     // Create an item to hold the origin label and shim allow indicator. Using
-    // an html element here, so we can use CSS flex rather than -moz-box, which
-    // handles the label overflow in combination with the icon correctly.
+    // an html element here, so we can use CSS flex, which handles the label
+    // overflow in combination with the icon correctly.
     let listItem = document.createElementNS(
       "http://www.w3.org/1999/xhtml",
       "div"