--- a/xpcom/glue/FileUtils.cpp
+++ b/xpcom/glue/FileUtils.cpp
@@ -445,17 +445,17 @@ mozilla::ReadAheadLib(mozilla::pathstr_t
 
   // The Mach header is followed by a sequence of load commands.
   // Each command has a header containing the command type and the
   // command size. LD_SEGMENT commands describes how the dynamic
   // loader is going to map the file in memory. We use that
   // information to find the biggest offset from the library that
   // will be mapped in memory.
   char *cmd = &base[sizeof(struct cpu_mach_header)];
-  off_t end = 0;
+  uint32_t end = 0;
   for (uint32_t ncmds = mh->ncmds; ncmds; ncmds--) {
     struct segment_command *sh = (struct segment_command *)cmd;
     if (sh->cmd != LC_SEGMENT) {
       continue;
     }
     if (end < sh->fileoff + sh->filesize) {
       end = sh->fileoff + sh->filesize;
     }