I'm trying out Ubuntu on some work PCs, but I'd like everything to be blocked on the users account, except Firefox and Flash Player. Essentially a parental controls application, but for the system/applications.
Easiest way: use a nonstandard window manager (fwm, twm, flwm...) and write a custom configuration file that starts Firefox maximized by default upon login, doesn't show anything else, and doesn't expose a menu for anything except Firefox or logoff.
If using GNOME: most menu icons come from /usr/share/applications, so delete (or make not world-readable) any that you don't want target user(s) to see icons for. Add back permissions for yourself (or other specific users) using "setfacl" or a "powerusers" group or something.
In GNOME, you'll also need to disable the "run..." key combination (otherwise a user could run "xterm", have a shell, and start anything). See https://unix.stackexchange.com/questions/27518/how-to-fully-lock-the-gnome-panel for how to lock the panel after you remove everything that could be used to escape to a shell.