diff --git a/mozglue/baseprofiler/core/platform.cpp b/mozglue/baseprofiler/core/platform.cpp
index 28783ed284d6..562a6aff3490 100644
--- a/mozglue/baseprofiler/core/platform.cpp
+++ b/mozglue/baseprofiler/core/platform.cpp
@@ -47,8 +47,10 @@
 #  include "mozilla/Atomics.h"
 #  include "mozilla/AutoProfilerLabel.h"
 #  include "mozilla/BaseProfilerDetail.h"
+#  include "mozilla/DoubleConversion.h"
 #  include "mozilla/Printf.h"
 #  include "mozilla/Services.h"
+#  include "mozilla/Span.h"
 #  include "mozilla/StackWalk.h"
 #  include "mozilla/StaticPtr.h"
 #  include "mozilla/ThreadLocal.h"
@@ -2422,35 +2424,36 @@ void profiler_init(void* aStackTop) {
 
     const char* startupDuration = getenv("MOZ_BASE_PROFILER_STARTUP_DURATION");
     if (startupDuration && startupDuration[0] != '\0') {
-      // TODO implement if needed
-      MOZ_CRASH("MOZ_BASE_PROFILER_STARTUP_DURATION unsupported");
-      // errno = 0;
-      // double durationVal = PR_strtod(startupDuration, nullptr);
-      // if (errno == 0 && durationVal >= 0.0) {
-      //   if (durationVal > 0.0) {
-      //     duration = Some(durationVal);
-      //   }
-      //   LOG("- MOZ_BASE_PROFILER_STARTUP_DURATION = %f", durationVal);
-      // } else {
-      //   LOG("- MOZ_BASE_PROFILER_STARTUP_DURATION not a valid float: %s",
-      //       startupDuration);
-      //   PrintUsageThenExit(1);
-      // }
+      // The duration is a floating point number. Use StringToDouble rather than
+      // strtod, so that "." is used as the decimal separator regardless of OS
+      // locale.
+      auto durationVal = StringToDouble(std::string(startupDuration));
+      if (durationVal && *durationVal >= 0.0) {
+        if (*durationVal > 0.0) {
+          duration = Some(*durationVal);
+        }
+        LOG("- MOZ_BASE_PROFILER_STARTUP_DURATION = %f", *durationVal);
+      } else {
+        LOG("- MOZ_BASE_PROFILER_STARTUP_DURATION not a valid float: %s",
+            startupDuration);
+        PrintUsageThenExit(1);
+      }
     }
 
     const char* startupInterval = getenv("MOZ_BASE_PROFILER_STARTUP_INTERVAL");
     if (startupInterval && startupInterval[0] != '\0') {
-      // TODO implement if needed
-      MOZ_CRASH("MOZ_BASE_PROFILER_STARTUP_INTERVAL unsupported");
-      // errno = 0;
-      // interval = PR_strtod(startupInterval, nullptr);
-      // if (errno == 0 && interval > 0.0 && interval <= 1000.0) {
-      //   LOG("- MOZ_BASE_PROFILER_STARTUP_INTERVAL = %f", interval);
-      // } else {
-      //   LOG("- MOZ_BASE_PROFILER_STARTUP_INTERVAL not a valid float: %s",
-      //       startupInterval);
-      //   PrintUsageThenExit(1);
-      // }
+      // The interval is a floating point number. Use StringToDouble rather than
+      // strtod, so that "." is used as the decimal separator regardless of OS
+      // locale.
+      auto intervalValue = StringToDouble(MakeStringSpan(startupInterval));
+      if (intervalValue && *intervalValue > 0.0 && *intervalValue <= 1000.0) {
+        interval = *intervalValue;
+        LOG("- MOZ_BASE_PROFILER_STARTUP_INTERVAL = %f", interval);
+      } else {
+        LOG("- MOZ_BASE_PROFILER_STARTUP_INTERVAL not a valid float: %s",
+            startupInterval);
+        PrintUsageThenExit(1);
+      }
     }
 
     features |= StartupExtraDefaultFeatures() & AvailableFeatures();
diff --git a/mozglue/misc/decimal/Decimal.cpp b/mozglue/misc/decimal/Decimal.cpp
index b08b4160f275..cc828e28439f 100644
--- a/mozglue/misc/decimal/Decimal.cpp
+++ b/mozglue/misc/decimal/Decimal.cpp
@@ -30,6 +30,7 @@
 
 #include "Decimal.h"
 #include "moz-decimal-utils.h"
+#include "DoubleConversion.h"
 
 using namespace moz_decimal_utils;
 
@@ -1048,3 +1049,15 @@ Decimal Decimal::zero(Sign sign)
 }
 
 } // namespace blink
+
+// Implementation of DoubleConversion.h:
+
+namespace mozilla {
+
+Maybe<double> StringToDouble(Span<const char> aStringSpan) {
+    bool valid = false;
+    double result = mozToDouble(aStringSpan, &valid);
+    return valid ? Some(result) : Nothing();
+}
+
+}
diff --git a/mozglue/misc/decimal/DoubleConversion.h b/mozglue/misc/decimal/DoubleConversion.h
new file mode 100644
index 000000000000..14c19e25400a
--- /dev/null
+++ b/mozglue/misc/decimal/DoubleConversion.h
@@ -0,0 +1,27 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A utility function that converts a string to a double independent of OS locale. */
+
+#ifndef MOZILLA_DOUBLECONVERSION_H
+#define MOZILLA_DOUBLECONVERSION_H
+
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+
+#include <string>
+
+namespace mozilla {
+
+// Parses aStringSpan into a double floating point value. Always treats . as the
+// decimal separator, regardless of OS locale. Consumes the entire string;
+// trailing garbage is invalid. Returns Nothing() for invalid input.
+// The implementation uses double_conversion::StringToDoubleConverter with
+// NO_FLAGS, see double-conversion/string-to-double.h for more documentation.
+Maybe<double> StringToDouble(Span<const char> aStringSpan);
+
+}
+
+#endif // MOZILLA_DOUBLECONVERSION_H
diff --git a/mozglue/misc/decimal/add-doubleconversion-impl.patch b/mozglue/misc/decimal/add-doubleconversion-impl.patch
new file mode 100644
index 000000000000..1cf0fb6ff199
--- /dev/null
+++ b/mozglue/misc/decimal/add-doubleconversion-impl.patch
@@ -0,0 +1,42 @@
+diff --git a/mozglue/misc/decimal/Decimal.cpp b/mozglue/misc/decimal/Decimal.cpp
+--- a/mozglue/misc/decimal/Decimal.cpp
++++ b/mozglue/misc/decimal/Decimal.cpp
+@@ -25,16 +25,17 @@
+  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+  */
+ 
+ #include "Decimal.h"
+ #include "moz-decimal-utils.h"
++#include "DoubleConversion.h"
+ 
+ using namespace moz_decimal_utils;
+ 
+ #include <algorithm>
+ #include <float.h>
+ 
+ namespace blink {
+ 
+@@ -1043,8 +1044,20 @@ bool Decimal::toString(char* strBuf, siz
+ }
+ 
+ Decimal Decimal::zero(Sign sign)
+ {
+     return Decimal(EncodedData(sign, EncodedData::ClassZero));
+ }
+ 
+ } // namespace blink
++
++// Implementation of DoubleConversion.h:
++
++namespace mozilla {
++
++Maybe<double> StringToDouble(Span<const char> aStringSpan) {
++    bool valid = false;
++    double result = mozToDouble(aStringSpan, &valid);
++    return valid ? Some(result) : Nothing();
++}
++
++}
diff --git a/mozglue/misc/decimal/moz-decimal-utils.h b/mozglue/misc/decimal/moz-decimal-utils.h
index a9caed0143b5..390bdaf02de0 100644
--- a/mozglue/misc/decimal/moz-decimal-utils.h
+++ b/mozglue/misc/decimal/moz-decimal-utils.h
@@ -14,6 +14,7 @@
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Casting.h"
 #include "mozilla/FloatingPoint.h"
+#include "mozilla/Span.h"
 
 #include <cmath>
 #include <cstring>
@@ -42,18 +43,22 @@
 
 typedef std::string String;
 
-double mozToDouble(const String &aStr, bool *valid) {
+double mozToDouble(mozilla::Span<const char> aStr, bool *valid) {
   double_conversion::StringToDoubleConverter converter(
     double_conversion::StringToDoubleConverter::NO_FLAGS,
     mozilla::UnspecifiedNaN<double>(), mozilla::UnspecifiedNaN<double>(), nullptr, nullptr);
-  const char* str = aStr.c_str();
-  int length = mozilla::AssertedCast<int>(strlen(str));
+  const char* str = aStr.Elements();
+  int length = mozilla::AssertedCast<int>(aStr.Length());
   int processed_char_count; // unused - NO_FLAGS requires the whole string to parse
   double result = converter.StringToDouble(str, length, &processed_char_count);
   *valid = mozilla::IsFinite(result);
   return result;
 }
 
+double mozToDouble(const String &aStr, bool *valid) {
+  return mozToDouble(mozilla::MakeStringSpan(aStr.c_str()), valid);
+}
+
 String mozToString(double aNum) {
   char buffer[64];
   int buffer_length = mozilla::ArrayLength(buffer);
diff --git a/mozglue/misc/decimal/update.sh b/mozglue/misc/decimal/update.sh
index 59e626f872b4..23748ebe2c25 100755
--- a/mozglue/misc/decimal/update.sh
+++ b/mozglue/misc/decimal/update.sh
@@ -35,6 +35,7 @@ if [ $# -eq 1 ]; then
     cp "$P" .
   done
 else
+  #LATEST_SHA=$(cat UPSTREAM-GIT-SHA)
   LATEST_SHA=$(git ls-remote https://chromium.googlesource.com/chromium/src.git/ | awk "/refs\/heads\/master/ {print \$1}")
   REPO_PATH="https://chromium.googlesource.com/chromium/src.git/+/$LATEST_SHA/third_party/WebKit/Source/platform"
   #REPO_PATH="https://github.com/WebKit/webkit/tree/master/Source/WebCore/platform"
@@ -49,10 +50,11 @@ fi
 
 # Apply patches:
 
-patch -p3 < zero-serialization.patch
-patch -p3 < comparison-with-nan.patch
-patch -p3 < mfbt-abi-markers.patch
-patch -p3 < to-moz-dependencies.patch
+patch -p4 < zero-serialization.patch
+patch -p4 < comparison-with-nan.patch
+patch -p4 < mfbt-abi-markers.patch
+patch -p4 < to-moz-dependencies.patch
+patch -p4 < add-doubleconversion-impl.patch
 # The following is disabled. See
 # https://bugzilla.mozilla.org/show_bug.cgi?id=1208357#c7
-#patch -p3 < fix-wshadow-warnings.patch
+#patch -p4 < fix-wshadow-warnings.patch
diff --git a/mozglue/misc/moz.build b/mozglue/misc/moz.build
index 247682cc0384..e859205f8ca1 100644
--- a/mozglue/misc/moz.build
+++ b/mozglue/misc/moz.build
@@ -9,6 +9,7 @@ FINAL_LIBRARY = 'mozglue'
 EXPORTS.mozilla += [
     'AutoProfilerLabel.h',
     'decimal/Decimal.h',
+    'decimal/DoubleConversion.h',
     'PlatformConditionVariable.h',
     'PlatformMutex.h',
     'Printf.h',