diff --git a/pkgs/applications/networking/browsers/firefox/common.nix b/pkgs/applications/networking/browsers/firefox/common.nix
index fb340ee1ace..732f9fa0872 100644
--- a/pkgs/applications/networking/browsers/firefox/common.nix
+++ b/pkgs/applications/networking/browsers/firefox/common.nix
@@ -120,8 +120,9 @@ buildStdenv.mkDerivation ({
   inherit src unpackPhase meta;
 
   patches = [
-    ./env_var_for_system_dir.patch
   ] ++
+  lib.optional (lib.versionOlder ffversion "86") ./env_var_for_system_dir-ff85.patch ++
+  lib.optional (lib.versionAtLeast ffversion "86") ./env_var_for_system_dir-ff86.patch ++
   lib.optional (lib.versionOlder ffversion "83") ./no-buildconfig-ffx76.patch ++
   lib.optional (lib.versionAtLeast ffversion "84") ./no-buildconfig-ffx84.patch ++
   lib.optional (ltoSupport && lib.versionOlder ffversion "84") ./lto-dependentlibs-generation-ffx83.patch ++
diff --git a/pkgs/applications/networking/browsers/firefox/env_var_for_system_dir.patch b/pkgs/applications/networking/browsers/firefox/env_var_for_system_dir-ff85.patch
similarity index 100%
rename from pkgs/applications/networking/browsers/firefox/env_var_for_system_dir.patch
rename to pkgs/applications/networking/browsers/firefox/env_var_for_system_dir-ff85.patch
diff --git a/pkgs/applications/networking/browsers/firefox/env_var_for_system_dir-ff86.patch b/pkgs/applications/networking/browsers/firefox/env_var_for_system_dir-ff86.patch
new file mode 100644
index 00000000000..f7a9cb77345
--- /dev/null
+++ b/pkgs/applications/networking/browsers/firefox/env_var_for_system_dir-ff86.patch
@@ -0,0 +1,21 @@
+diff -r 22fc47c968f2 toolkit/xre/nsXREDirProvider.cpp
+--- a/toolkit/xre/nsXREDirProvider.cpp	Mon Dec 14 15:09:17 2020 +0000
++++ b/toolkit/xre/nsXREDirProvider.cpp	Tue Feb 23 23:38:56 2021 +0100
+@@ -11,6 +11,7 @@
+ 
+ #include "jsapi.h"
+ #include "xpcpublic.h"
++#include "prenv.h"
+ 
+ #include "nsIAppStartup.h"
+ #include "nsIFile.h"
+@@ -305,7 +306,8 @@
+       "/usr/lib/mozilla"_ns
+ #    endif
+       ;
+-  rv = NS_NewNativeLocalFile(dirname, false, getter_AddRefs(localDir));
++  const char* pathVar = PR_GetEnv("MOZ_SYSTEM_DIR");
++  rv = NS_NewNativeLocalFile((pathVar && *pathVar) ? nsDependentCString(pathVar) : reinterpret_cast<const nsCString&>(dirname), false, getter_AddRefs(localDir));
+ #  endif
+ 
+   if (NS_SUCCEEDED(rv)) {
diff --git a/pkgs/applications/networking/browsers/firefox/packages.nix b/pkgs/applications/networking/browsers/firefox/packages.nix
index 86d91d21e3c..1bcaecdfbb8 100644
--- a/pkgs/applications/networking/browsers/firefox/packages.nix
+++ b/pkgs/applications/networking/browsers/firefox/packages.nix
@@ -7,10 +7,10 @@ in
 rec {
   firefox = common rec {
     pname = "firefox";
-    ffversion = "85.0.2";
+    ffversion = "86.0";
     src = fetchurl {
       url = "mirror://mozilla/firefox/releases/${ffversion}/source/firefox-${ffversion}.source.tar.xz";
-      sha512 = "2m46li5ni1m4xv42h99rn2hhhv2mqy229wihmzxmgvws1rh2h11yf6x2a07akkjrsp2dmwxmmkhmf9dhakgj9i55z5qqi99azyx07df";
+      sha512 = "f5a7b9aa53e2955e972e30cc62ae64ae955127eab951d7a1102d5ac8a73804982e01b917787c00a6e0a0ed03594567d29a24808271283ef61a9383bc4bb39e3f";
     };
 
     meta = {