package.json: add Firefox in browserslist section.

Firefox v38 is older than the ESR, so basically replace ESR with v38.

The dist file change with the Firefox >= 38 addition is the following:

 dist/css/bootstrap.css | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/dist/css/bootstrap.css b/dist/css/bootstrap.css
index e37b3d46c..c93e561cd 100644
--- a/dist/css/bootstrap.css
+++ b/dist/css/bootstrap.css
@@ -1874,6 +1874,11 @@ pre code {
   opacity: 1;
 }
 
+.form-control::-moz-placeholder {
+  color: #868e96;
+  opacity: 1;
+}
+
 .form-control:-ms-input-placeholder {
   color: #868e96;
   opacity: 1;
@@ -4406,8 +4411,10 @@ tbody.collapse.show {
 @media (min-width: 576px) {
   .card-columns {
     -webkit-column-count: 3;
+    -moz-column-count: 3;
     column-count: 3;
     -webkit-column-gap: 1.25rem;
+    -moz-column-gap: 1.25rem;
     column-gap: 1.25rem;
   }
   .card-columns .card {

Fixes #24911 (closed), fixes #25017 (closed).