You can install both as they do different things. My recommendation is for everyone to use only Firefox with these two extensions installed while using a public network. Anything else will leave your account vulnerable to mischief.
However, using these add-ons will slow down your browser and limit some functions. Facebook chat will not work at all with these installed.
Techcrunch reported the release of Firesheep has received over 104,000 downloads and climbing. This easy to install Firefox plugin allows users to gain access to virtually every popular social networking site of another user on an unsecured wifi network at the click of a button. The hack was developed by Eric Butler to bring this long standing vulnerability to wide public awareness. Butler stated in his site:
This is a widely known problem that has been talked about to death, yet very popular websites continue to fail at protecting their users. The only effective fix for this problem is full end-to-end encryption, known on the web as HTTPS or SSL. Facebook is constantly rolling out new “privacy” features in an endless attempt to quell the screams of unhappy users, but what’s the point when someone can just take over an account entirely? Twitter forced all third party developers to use OAuth then immediately released (and promoted) a new version of their insecure website.Firesheep is free, open source, and is available now for Mac OS X and Windows. Linux support is on the way.Websites have a responsibility to protect the people who depend on their services. They’ve been ignoring this responsibility for too long, and it’s time for everyone to demand a more secure web. My hope is that Firesheep will help the users win.