You can limit the downlink of a Firefox browser to 512 KB/s with this command:
trickle -d 512 /usr/bin/firefox -no-remote -P test
More informations can be found on these pages:
Usage: trickle [-hvVs] [-d <rate>] [-u <rate>] [-w <length>] [-t <seconds>] [-l <length>] [-n <path>] command ... -h Help (this) -v Increase verbosity level -V Print trickle version -s Run trickle in standalone mode independent of trickled -d <rate> Set maximum cumulative download rate to <rate> KB/s -u <rate> Set maximum cumulative upload rate to <rate> KB/s -w <length> Set window length to <length> KB -t <seconds> Set default smoothing time to <seconds> s -l <length> Set default smoothing length to <length> KB -n <path> Use trickled socket name <path> -L <ms> Set latency to <ms> milliseconds -P <path> Preload the specified .so instead of the default one