diff --git a/media/liboggz/src/liboggz/oggz_seek.c b/media/liboggz/src/liboggz/oggz_seek.c
--- a/media/liboggz/src/liboggz/oggz_seek.c
+++ b/media/liboggz/src/liboggz/oggz_seek.c
@@ -614,6 +614,12 @@ oggz_offset_end (OGGZ * oggz)
   return offset_end;
 }
 
+static int
+is_header_page(ogg_page* page)
+{
+  return page && page->header[5] & 0x2;
+}
+
 ogg_int64_t
 oggz_bounded_seek_set (OGGZ * oggz,
                        ogg_int64_t unit_target,
@@ -683,8 +689,18 @@ oggz_bounded_seek_set (OGGZ * oggz,
   if (unit_begin == -1 && oggz_seek_raw (oggz, offset_begin, SEEK_SET) >= 0) {
     ogg_int64_t granulepos = 0;
     unit_begin = 0;
-    // Start time needs to be the end time of the first non-header page.
-    while (oggz_get_next_start_page (oggz, og) >= 0 && unit_begin <= 0) {
+    
+    // Take the start time of the range as the end time of the next page. Note
+    // that if unit_target lies inside that page, its timestamp will be less
+    // than the page's end time, and it will be considered outside of the range.
+    // If the next page is a header page, we're at the start of the media,
+    // and in such a case we can't take the next content-page's granulepos's
+    // timestamp as unit_begin, as if the target lies inside this page, we'll
+    // miss it. Assume unit_begin is 0 in that case. See Mozilla bug 518169.
+    while (oggz_get_next_start_page (oggz, og) >= 0 &&
+           !is_header_page(og) &&
+           unit_begin <= 0)
+    {
       serialno = ogg_page_serialno (og);
       granulepos = ogg_page_granulepos (og);
       unit_begin = oggz_get_unit (oggz, serialno, granulepos);
@@ -693,6 +709,7 @@ oggz_bounded_seek_set (OGGZ * oggz,
 
   /* Fail if target isn't in specified range. */
   if (unit_target < unit_begin || unit_target > unit_end) {
+    oggz_reset (oggz, offset_orig, unit_at, SEEK_SET);
     return -1;
   }
 
@@ -773,6 +790,10 @@ oggz_bounded_seek_set (OGGZ * oggz,
 
    /* Reader is now approximately at the seek target. */
 
+  offset_at = oggz_reset (oggz, offset_at, unit_at, SEEK_SET);
+  if (offset_at == -1)
+    return -1;
+
   return (long)reader->current_unit;
 }