diff -up firefox-84.0/security/sandbox/linux/moz.build.1516803 firefox-84.0/security/sandbox/linux/moz.build

--- firefox-84.0/security/sandbox/linux/moz.build.1516803	2020-12-10 16:17:55.425139545 +0100

+++ firefox-84.0/security/sandbox/linux/moz.build	2020-12-10 16:29:21.945860841 +0100

@@ -114,9 +114,8 @@ if CONFIG["CC_TYPE"] in ("clang", "gcc")

 # gcc lto likes to put the top level asm in syscall.cc in a different partition

 # from the function using it which breaks the build.  Work around that by

 # forcing there to be only one partition.

-for f in CONFIG["OS_CXXFLAGS"]:

-    if f.startswith("-flto") and CONFIG["CC_TYPE"] != "clang":

-        LDFLAGS += ["--param lto-partitions=1"]

+if CONFIG['CC_TYPE'] != 'clang':

+    LDFLAGS += ['--param', 'lto-partitions=1']

 DEFINES["NS_NO_XPCOM"] = True

 DisableStlWrapping()