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()