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:

• trickle Homepage
• Trickle in the Ubunutuuser Wiki (german)

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