--- a/hgext/firefoxtree/__init__.py
+++ b/hgext/firefoxtree/__init__.py
@@ -141,17 +141,17 @@ def isfirefoxrepo(repo):
 
     try:
         if len(repo) and repo[0].hex() in (MOZ_ROOT_REV, COMM_ROOT_REV):
             return True
     except error.FilteredRepoLookupError:
         pass
 
     # Backdoor for testing.
-    return repo.opener.exists('IS_FIREFOX_REPO')
+    return repo.vfs.exists('IS_FIREFOX_REPO')
 
 # Wrap repo lookup to automagically resolve tree names to URIs.
 def peerorrepo(orig, ui, path, *args, **kwargs):
     try:
         return orig(ui, path, *args, **kwargs)
     except RepoError:
         tree, uri = resolve_trees_to_uris([path])[0]
         if not uri:
@@ -225,17 +225,17 @@ def writefirefoxtrees(repo):
 
     with open(repo._firefoxtreespath, 'wb') as fh:
         fh.write('\n'.join(lines))
 
     # Old versions of firefoxtrees stored labels in the localtags file. Since
     # this file is read by Mercurial and has no relevance to us any more, we
     # prune relevant entries from this file so the data isn't redundant with
     # what we now write.
-    localtags = repo.opener.tryread('localtags')
+    localtags = repo.vfs.tryread('localtags')
     havedata = len(localtags) > 0
     taglines  = []
     for line in localtags.splitlines():
         line = line.strip()
         node, tag = line.split()
         tree, uri = resolve_trees_to_uris([tag])[0]
         if not uri:
             taglines.append(line)