diff --git a/pkgs/applications/networking/mailreaders/thunderbird/packages.nix b/pkgs/applications/networking/mailreaders/thunderbird/packages.nix
index 43117b3af188..adb295c8c682 100644
--- a/pkgs/applications/networking/mailreaders/thunderbird/packages.nix
+++ b/pkgs/applications/networking/mailreaders/thunderbird/packages.nix
@@ -4,30 +4,11 @@
   buildMozillaMach,
   callPackage,
   fetchurl,
-  icu77,
-  icu78,
   fetchpatch2,
   config,
 }:
 
 let
-  patchICU =
-    icu:
-    icu.overrideAttrs (attrs: {
-      # standardize vtzone output
-      # Work around ICU-22132 https://unicode-org.atlassian.net/browse/ICU-22132
-      # https://bugzilla.mozilla.org/show_bug.cgi?id=1790071
-      patches = attrs.patches ++ [
-        (fetchpatch2 {
-          url = "https://hg.mozilla.org/mozilla-central/raw-file/fb8582f80c558000436922fb37572adcd4efeafc/intl/icu-patches/bug-1790071-ICU-22132-standardize-vtzone-output.diff";
-          stripLen = 3;
-          hash = "sha256-MGNnWix+kDNtLuACrrONDNcFxzjlUcLhesxwVZFzPAM=";
-        })
-      ];
-    });
-  icu77' = patchICU icu77;
-  icu78' = patchICU icu78;
-
   common =
     {
       version,
@@ -48,10 +29,6 @@ let
         # The file to be patched is different from firefox's `no-buildconfig-ffx90.patch`.
         (if lib.versionOlder version "140" then ./no-buildconfig.patch else ./no-buildconfig-tb140.patch)
       ];
-      extraPassthru = {
-        icu77 = icu77';
-        icu78 = icu78';
-      };
 
       meta = {
         changelog = "https://www.thunderbird.net/en-US/thunderbird/${version}/releasenotes/";
@@ -76,9 +53,6 @@ let
         webrtcSupport = false;
 
         pgoSupport = false; # console.warn: feeds: "downloadFeed: network connection unavailable"
-
-        icu77 = icu77';
-        icu78 = icu78';
       };
 
 in
diff --git a/pkgs/build-support/build-mozilla-mach/default.nix b/pkgs/build-support/build-mozilla-mach/default.nix
index 183f717d6c4b..1236f7b79369 100644
--- a/pkgs/build-support/build-mozilla-mach/default.nix
+++ b/pkgs/build-support/build-mozilla-mach/default.nix
@@ -76,8 +76,6 @@ in
   glib,
   gnum4,
   gtk3,
-  icu77, # if you fiddle with the icu parameters, please check Thunderbird's overrides
-  icu78,
   libGL,
   libGLU,
   libevent,
@@ -359,10 +357,6 @@ buildStdenv.mkDerivation {
     rm -rf obj-x86_64-pc-linux-gnu
     patchShebangs mach build
   ''
-  # https://bugzilla.mozilla.org/show_bug.cgi?id=1927380
-  + lib.optionalString (lib.versionAtLeast version "134") ''
-    sed -i "s/icu-i18n/icu-uc &/" js/moz.configure
-  ''
   + extraPostPatch;
 
   # Ignore trivial whitespace changes in patches, this fixes compatibility of
@@ -506,7 +500,8 @@ buildStdenv.mkDerivation {
     # MacOS builds use bundled versions of libraries: https://bugzilla.mozilla.org/show_bug.cgi?id=1776255
     "--enable-system-pixman"
     "--with-system-ffi"
-    "--with-system-icu"
+    # Mozilla vendors 10+ patches and ICU upstream is very slow to adopt them
+    # "--with-system-icu"
     "--with-system-jpeg"
     "--with-system-libevent"
     "--with-system-libvpx"
@@ -609,7 +604,6 @@ buildStdenv.mkDerivation {
       libdrm
     ]
   ))
-  ++ [ (if (lib.versionAtLeast version "147") then icu78 else icu77) ]
   ++ lib.optional gssSupport libkrb5
   ++ lib.optional jemallocSupport jemalloc
   ++ extraBuildInputs;