Improve Liberapay css for Firefox compatibility

src/sass/navbar.scss

@@ -70,8 +70,9 @@ nav {
 
 .lp {
     height: 14px;
-    margin-top: 2px;
-    display: block;
+    display: inline-block;
+    position: relative;
+    top: 2px;
     fill: var(--fg_nav);
 
     &:hover {