diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 025b767d7d01..2586aabcd4cc 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1281,7 +1281,9 @@ FireForgetSkippable(uint32_t aSuspected, bool aRemoveChildless,
     if (!aDeadline.IsNull()) {
       if (aDeadline < now) {
         // This slice overflowed the idle period.
-        idleDuration = aDeadline - startTimeStamp;
+        if (aDeadline > startTimeStamp) {
+          idleDuration = aDeadline - startTimeStamp;
+        }
       } else {
         idleDuration = duration;
       }