You have 2 easy to use via terminal. Wondershaper and Trickle. There are more here: How to limit internet speed?
Anyway the basics are:
sudo apt-get install wondershaper
In terminal find out what is the name of the interface connected to your internet by using
ifconfig. Could be eth0, eth1, eth2, etc... In my case is eth1. So, I would do this:
sudo wondershaper eth1 1024 256
This will give me a Download speed of 1024 Kbits and 256 Kbits Upload. Which are 128KB and 16KB.
If you want to reset that do this
sudo wondershaper clear eth1
NOTE: That will limit ANYTHING in your PC, for example the torrent downloader, firefox, etc..
Another program is Trickle
To install do this:
sudo apt-get install trickle
firefox with a maximum of 120KB/s download and 32KB/s upload, run
trickle -d 120 -u 32 firefox
In my case I recommend wondershaper.
If you want more info for any of the two after installed type
man wondershaper or
man trickle depending on the one you installed.