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