Merge Firefox security changes from Debian Buster

browser/app/profile/firefox.js

@@ -1053,7 +1053,7 @@ pref("dom.ipc.plugins.sandbox-level.flash", 0);
 // On windows these levels are:
 // See - security/sandbox/win/src/sandboxbroker/sandboxBroker.cpp
 // SetSecurityLevelForContentProcess() for what the different settings mean.
-pref("security.sandbox.content.level", 5);
+pref("security.sandbox.content.level", 6);
 
 // This controls the depth of stack trace that is logged when Windows sandbox
 // logging is turned on.  This is only currently available for the content