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

To run 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.