diff --git a/pkgs/countfftabs/count.cpp b/pkgs/countfftabs/count.cpp
new file mode 100644
index 0000000..b771342
--- /dev/null
+++ b/pkgs/countfftabs/count.cpp
@@ -0,0 +1,55 @@
+#include <iostream>
+#include <fstream>
+#include <endian.h>
+
+#include <simdjson.h>
+#include <lz4.h>
+using namespace simdjson;
+// /home/yorick/.mozilla/firefox/sdgle03g.default/sessionstore-backups/recovery.jsonlz4
+// jq '.windows | map(.tabs | length) | add'
+
+padded_string read_mozlz4a_file(const std::string& file) {
+  std::ifstream is(file, std::ifstream::binary);
+  if (!is) {
+    std::cerr << "error opening file" << std::endl;
+    exit(1);
+  }
+  is.seekg(0, is.end);
+  size_t insz = is.tellg();
+  is.seekg(0, is.beg);
+  char * buffer = new char[insz];
+  is.read(buffer, insz);
+  if (!is) {
+    std::cerr << "error reading file" << std::endl;
+    exit(1);
+  }
+  is.close();
+  if (memcmp(buffer, "mozLz40", 8)) {
+    std::cerr << "not a mozLZa file" << std::endl;
+    exit(1);
+  }
+  size_t outsz = le32toh(*(uint32_t *) (buffer + 8));
+  padded_string out(outsz);
+  if (LZ4_decompress_safe_partial(buffer + 12, out.data(), insz - 12, outsz, outsz) < 0) {
+    std::cerr << "decompression error" << std::endl;
+    exit(1);
+  }
+  return out;
+}
+
+int main(int ac, char **av) {
+    ondemand::parser parser;
+    if (ac < 2) {
+      std::cerr << "usage: " << av[0] << " .mozilla/firefox/*.default/sessionstore-backups/recovery.jsonlz4" << std::endl;
+      exit(1);
+    }
+    padded_string json = read_mozlz4a_file(av[1]);
+    ondemand::document session = parser.iterate(json);
+    ondemand::array windows = session["windows"];
+    size_t n = 0;
+    for (auto i : windows) {
+      ondemand::array tabs = i["tabs"];
+      n += tabs.count_elements();
+    }
+    std::cout << n << std::endl;
+}
diff --git a/pkgs/countfftabs/default.nix b/pkgs/countfftabs/default.nix
new file mode 100644
index 0000000..d71f9da
--- /dev/null
+++ b/pkgs/countfftabs/default.nix
@@ -0,0 +1,14 @@
+{
+  stdenv, simdjson, lz4
+}:
+stdenv.mkDerivation {
+  name = "countfftabs";
+  src = ./.;
+  buildInputs = [simdjson lz4];
+  buildPhase = ''
+    runHook preBuild
+    g++ -o ./countfftabs count.cpp -I${simdjson}/include -L${simdjson}/lib -lsimdjson -llz4
+    runHook postBuild
+  '';
+  installPhase = "mkdir -p $out/bin; mv ./countfftabs $out/bin/";
+}
diff --git a/pkgs/default.nix b/pkgs/default.nix
index fa65623..12fb31e 100644
--- a/pkgs/default.nix
+++ b/pkgs/default.nix
@@ -15,5 +15,23 @@
   playerctl = super.playerctl.overrideAttrs (o: {
     patches = (o.patches or []) ++ [ ./playerctl-solid-emoji.diff ];
   });
+  countfftabs = super.callPackage ./countfftabs {};
+  lz4json = super.stdenv.mkDerivation (o: {
+    pname = "lz4json";
+    version = "20191229";
+    src = super.fetchFromGitHub {
+      repo = o.pname;
+      owner = "andikleen";
+      rev = "c44c51005c505de2636cc1e59cde764490de7632";
+      hash = "sha256-rLjJ7qy7Tx0htW1VxrfCCqVbC6jNCr9H2vdDAfosxCA=";
+    };
+    buildInputs = [ super.lz4 ];
+    nativeBuildInputs = [ super.pkg-config ];
+    installPhase = ''
+      runHook preInstall
+      install -D -t $out/bin lz4jsoncat
+      runHook postInstall
+    '';
+  });
 
 })