This has been possible for a long time (some tcpdump filters and some cookie setting scripts and you're golden), but this tool makes it insanely easy. You can check out the tool and its description on the main site.
The site mentions that Linux support is on the way, and recently it has become possible to successfully compile the tool on Linux. This is what I did to get it running on Ubuntu 10.10.
cd ~git clone git://github.com/mickflemm/firesheep.git
This will configure the makefile, this will probably fail until you have installed all the right dependencies. Below are the ones I was missing, yours might differ! Check the output for hints on which package you need to install.
sudo apt-get install libxul-dev xulrunner-devsudo libpcap-devsudo libhal-dev
# Now configure should succeed:
We now need to make a small change to the makefile.
Change the MOZ_CFLAGS = ... line to:
MOZ_CFLAGS = -fshort-wchar -I/usr/lib/xulrunner-devel-184.108.40.206/include -I/usr/include/nspr
Now we start compiling.
This will probably fail with the message:
make: *** No rule to make target `deps/http-parser/http_parser.c', needed by `../xpi/platform/.../firesheep-backend'. Stop.
This is fixed by running a submodule update:
git submodule update --init
And make again:
You'll now have an extension in the build directory. Drag the .xpi to Firefox to install Firesheep, then close Firefox completely.
Firesheep expects your wireless interface to be in monitor mode. The easiest way to do this is to use airmon-ng:
sudo airmon-ng start eth1 #Substitute your wireless interface name
Now start Firefox with root rights:
Go to Tools → Add-ons, and open the Firesheep Preferences under the Extensions view. Another window opens. Set the Capture Interface to mon0.
Press CTRL+SHIFT+S to open the Firesheep sidebar and to start capturing.
Good luck. Also, be sure to check out Blacksheep, a tool to detect Firesheep tampering on your network.