Can I limit my SSH Tunneling Speed?
I'm not familiar with DownThemAll, but I see that it's a Firefox extension. So you could try running trickle to launch Firefox with a download limit (see man trickle). For example:
trickle -d 20 firefox
That will set a download limit of 20 KB/s.
For completeness, users running Windows can do the same by using the Firefox Throttle add-on.