Popup menus have no border or padding, causing the first item to be clicked automatically

Firefox version: Firefox Developer Edition 61.0b10 (64-bit) Operating System: Linux Installed shadowfox: (Installed via Arch AUR)

› shadowfox-updater --version
ShadowFox updater 1.5.1
› pacman -Q shadowfox-updater 
shadowfox-updater 1.5.1-1

When I right click anywhere on the page, the first menu item is automatically chosen because it ends up right under the cursor. I can only use right click menus by holding down right click and then move the cursor to the item I want before I release.

This happens because Shadowfox removes border and padding of popup menus. I don't understand why it does that; shouldn't it only customize colors?

I fixed it by commenting out the changes to size and behavior.

diff --git a/userChrome-original.css b/userChrome-fixed.css
index 25b6901..77451b9 100644
--- a/userChrome-original.css
+++ b/userChrome-fixed.css
@@ -133,10 +133,10 @@ menupopup > menu > menupopup,
 menupopup scrollbox,
 popup,
 popup > menu > menupopup {
-  -moz-appearance: none!important;
+  /* -moz-appearance: none!important; */
   background: var(--in-content-box-background)!important;
-  border: none!important;
-  padding: 0!important
+  /* border: none!important; */
+  /* padding: 0!important */
 }
 menu.subviewbutton > .menu-right {
   fill: #000!important
@@ -154,16 +154,16 @@ menu.subviewbutton > .menu-right {
 }
 menuitem,
 menupopup menu {
-  -moz-appearance: none!important;
+  /* -moz-appearance: none!important; */
   color: var(--in-content-selected-text)!important;
   background: var(--in-content-box-background)!important
 }
 menupopup menuseparator {
-  -moz-appearance: none!important;
-  padding: 1px!important;
-  margin: 5px 0!important;
+  /* -moz-appearance: none!important; */
+  /* padding: 1px!important; */
+  /* margin: 5px 0!important; */
   background: var(--in-content-table-border-dark-color)!important;
-  border-top: none!important
+  /* border-top: none!important */
 }
 #context-navigation menuitem[disabled=true],
 menu[disabled=true],

Screenshot before

Mouse cursor is on the origin pixel of the first item (0×0) so it is selected on button release.

Screenshot after

Mouse cursor ends up on the border so nothing is selected.