about:startup - PR_IntervalNow is kinda dumb. It makes no special attempt to be monotonic like the docs say, and additionally there's no way to equate it back to a real timestamp (which the docs do imply). Using PR_Now instead.

browser/base/content/aboutStartup.js

@@ -4,4 +4,4 @@ var Ci = Components.interfaces;
 var runtime = Cc["@mozilla.org/xre/runtime;1"]
                 .getService(Ci.nsIXULRuntime);
 var started = document.getElementById("started");
-started.textContent = runtime.startupTimestamp
+started.textContent = new Date(runtime.startupTimestamp/1000) +" ("+ runtime.startupTimestamp +")";

toolkit/xre/nsAppData.cpp

@@ -73,7 +73,7 @@ ScopedAppData::ScopedAppData(const nsXREAppData* aAppData)
   this->size = aAppData->size;
 
   if (aAppData->size > offsetof(nsXREAppData, startupTimestamp)) {
-    this->startupTimestamp = PR_IntervalNow();
+    this->startupTimestamp = PR_Now();
   }
 
   SetAllocatedString(this->vendor, aAppData->vendor);

toolkit/xre/nsAppRunner.cpp

@@ -806,13 +806,13 @@ nsXULAppInfo::InvalidateCachesOnRestart()
 }
 
 /* readonly attribute unsigned long launchTimestamp; */
-NS_IMETHODIMP nsXULAppInfo::GetLaunchTimestamp(PRUint32 *aTimestamp)
+NS_IMETHODIMP nsXULAppInfo::GetLaunchTimestamp(PRUint64 *aTimestamp)
 {
     return NS_ERROR_NOT_IMPLEMENTED;
 }
 
 /* readonly attribute unsigned long startupTimestamp; */
-NS_IMETHODIMP nsXULAppInfo::GetStartupTimestamp(PRUint32 *aTimestamp)
+NS_IMETHODIMP nsXULAppInfo::GetStartupTimestamp(PRUint64 *aTimestamp)
 {
   *aTimestamp = gAppData->startupTimestamp;
   return NS_OK;

xpcom/build/nsXULAppAPI.h

@@ -155,7 +155,7 @@ struct nsXREAppData
   /**
    * 
    */
-  PRIntervalTime startupTimestamp;
+  PRTime startupTimestamp;
 };
 
 /**

xpcom/system/nsIXULRuntime.idl

@@ -108,6 +108,6 @@ interface nsIXULRuntime : nsISupports
    */
   void invalidateCachesOnRestart();
 
-  readonly attribute unsigned long launchTimestamp;
-  readonly attribute unsigned long startupTimestamp;
+  readonly attribute unsigned long long launchTimestamp;
+  readonly attribute unsigned long long startupTimestamp;
 };