Well let's say that if you have a portable version of the Firefox, then you can have a dedicated SSH client with you. But secondly and most importantly, let's say that the PC on your work-place does not allow you to install applications but allows you to add browser pluggings, then you have the ability to install and sue FireSSH with your browser and access some of those applications via the Terminal window! (as long as those ports are open for your browser of course).
So it is pretty handy to have around. If you want to install it then again make sure you have Firefox4 (still in beta) installed and go to this page and click on the "add to Firefox" button and follow the instructions on the screen.