Yes, of course, Firefox has had a "fullscreen" mode called by F11 ever since day 1. But it has a few shortcomings, like :
- when it hides your statusbar, it hides it for good, you need to get out of fullscreen mode to see it,
- if you have several tabs open, it doesn't hide the tab bar, thus not "true" full screen.
The plugin is meant to improve on the existing feature, that's why it is called "fullERscreen", and reuses the F11 key.
What it does is, it hides everything including the tab bar, but if your mouse comes near the top or bottom of screen, the tab bar and status bar pop up (so you can change tabs or click on something on the status bar), then disappear again when the mouse moves away. There is also an option where the status bar pops up if something changes (e.g. Gmail notifier).
It may sound like a small change but it makes a lot of difference, especially on a 800*480 screen like the SA1 has.