I use Firefox for anything on the web. Since I primarily use a Mac and sometimes a PC at work, I like to have the same browser on both machines. The UI is the same and there are some extensions that I've grown quite accustomed to.
Firefox wasn't officially supported in PeopleSoft until PeopleTools 8.46, but there were only a few pages that didn't work for me prior to 8.46: ePro > Create Req in Fin 8.8, REN Server. Now that Firefox is officially supported in, I only use Firefox when working in PeopleSoft.
There are a few nuances that Firefox has I want to point out. First, the CNTRL- J combo that PeopleSoft uses to show you the Component and Page information is also the keyboard shortcut for the download manager. There are two ways around this: CNTRL-J-J (double J - very quickly), and CNTRL-SHIFT-J. Choose your poison.
Tabbed browsing in Firefox is great, but tabs don't work well with PeopleSoft. I have Firefox setup to always create a new tab when a link is set to create a new window (such as the "New Window" link in PeopleSoft). Firefox does not handle session cookies in tabs the same as it does for separate windows. So, if you have two PeopleSoft tabs for the same environment, or different environments, the session cookies will invalidate each other.
The Firefox 2 beta seems to have fixed the problem. I have been able to work in different envrionments using tabs without any errors. Hopefully, the problem is fixed.