Add firefox wrapper

Now, this is *really* ugly - I'm exploiting the fact that firefox
respects $HOME for determining the user's home directory. Don't try this
at... home! (*ba-dum-ts*)

I also log every call to this script. The idea is to get a usage
frequency to determine at what point I'm safe to uninstall firefox
without causing any headaches later on (e.g. when the "last use"
timestamp dates back several months).

bin/firefox

@@ -0,0 +1,15 @@
+#!/usr/bin/env sh
+. "$(dirname "$0")"/path.sh
+
+# Set up data directory:
+MOZDIR="$XDG_DATA_HOME"/mozilla
+if [ ! -d "$MOZDIR" ]; then
+	mkdir "$MOZDIR"
+	ln -s "$HOME"/tmp "$MOZDIR"/tmp # TODO XDG_DOWNLOAD_DIR, don't hardcode
+fi
+
+# Log "last used" (for statistics):
+mkdir -p "$XDG_LOG_HOME"/mozilla
+date -Iseconds >> "$XDG_LOG_HOME"/mozilla/usage.log
+
+HOME="$XDG_DATA_HOME"/mozilla exec "$EPATH" "$@"