diff --git a/support/gecko/nsstring/src/lib.rs b/support/gecko/nsstring/src/lib.rs
index 02352212938..8e96b9ef827 100644
--- a/support/gecko/nsstring/src/lib.rs
+++ b/support/gecko/nsstring/src/lib.rs
@@ -553,6 +553,9 @@ macro_rules! define_string_types {
             hdr: $StringRepr,
         }
 
+        unsafe impl Send for $String {}
+        unsafe impl Sync for $String {}
+
         impl $String {
             pub fn new() -> $String {
                 $String {