After connection you'll be in the context of the browser window, i.e. all the code you'll type will be evaluated there:

repl> repl.whereAmI()
[object ChromeWindow] - Document title: "Firefox"
repl> repl.look()
this.repl=[object]
this.browserContentListener=[object]
this.getInterface=[function]
this.XULBrowserWindow=[object]
this.tryToClose=[function]
this.document=[object]
[...]

So typing title will be like typing window.title:

repl> title
Firefox

You can modify what you see around, of course. The following changes the window title and hides the toolbar:

repl> title = 'REPL-powered Firefox'
REPL-powered Firefox
repl> document.getElementById('toolbar-menubar').hidden = true
true

You can enter other parts of the interface, have a look, play with things, get online docs for XUL elements (a browser popup appears), then go back:

repl> var c = document.getElementById('urlbar-container')
repl> repl.enter(c)
[object XULElement]
repl> firstChild
[object XULElement]
repl> repl.doc(firstChild)
TYPE: object
NODENAME: textbox

Online help found, displaying...
repl> repl.back()
[object ChromeWindow]

When your curiosity about the browser's insides is satisfied, you might start exploring web pages:

repl> repl.home()
[object ChromeWindow]
repl> content.location.href = 'http://maps.google.com'
http://www.google.com
repl> repl.enter(content) // for Firefox 3 you'll need: repl.enter(content.wrappedJSObject)
[object Window]
repl> repl.whereAmI()
[object Window] - Document title: "Google Maps"
repl> repl.look()
this.G_HYBRID_TYPE=[object]
this.G_HYBRID_MAP=[object]
this.G_SATELLITE_TYPE=[object]
this.G_SATELLITE_MAP=[object]
this.G_MAP_TYPE=[object]
this.G_NORMAL_MAP=[object]
this.G_DEFAULT_MAP_TYPES=[probably array, length 3]
this.GAddCopyright=[function]
this.GSmartPaste=[function]
this.GSendToPhone=[function]
[...]
repl> repl.search(/app/i)
GMapsApplication
loadApplication
gApplication
_mMapPrintUrl
XPCNativeWrapper
repl> repl.doc(gApplication)
TYPE: object

repl> repl.enter(gApplication)
[object Object]
repl> repl.look()
this.ra=[object]
this.We=[object]
this.a=[object]
this.Xf=[object]
this.Ma=[object]
[...]
repl> repl.search(/get/)
getMap
getOverviewMapControl
getPageUrl
getTabUrl
getMarker
getPolyline
getVPageUrlParams
repl> repl.doc(getPageUrl)
TYPE: function
ARGS: [none declared]

repl> getPageUrl()
http://maps.google.com/?ll=37.020098,-49.042969&spn=61.328812,93.164062&om=1
repl>

To go back to the context where the REPL was started:

repl> repl.home();
[object ChromeWindow]
repl>

Creating a new context and working inside it, e.g. for testing new stuff, is easy, and what you define there, stays there:

repl> var scratch = {}
repl> repl.enter(scratch)
[object Object]    
repl> repl.look()
this is empty
repl> var x = 2
repl> var y = 'hello'
repl> x
2
repl> y
hello
repl> repl.look()
this.x=2
this.y=hello
repl> repl.back()
[object ChromeWindow]
repl> x
	@data:application/x-javascript,x:1

!!! ReferenceError: x is not defined

repl> y
	@data:application/x-javascript,y:1

!!! ReferenceError: y is not defined

repl> scratch.x
2
repl> scratch.y
hello
repl> 

Of course, you can also mess with the REPL itself:

repl> repl.enter(repl)
[object Object]
repl> function sayHello() { this.print('hello from repl!') }
repl> sayHello.doc = 'just say hello'
just say hello
repl> repl.back()
[object ChromeWindow]
repl> repl.doc(repl.sayHello)
TYPE: function
NAME: sayHello
ARGS: [none declared]

just say hello

repl> repl.sayHello()
hello from repl!

To find out more about REPL commands, inspect() it, and then use doc() for more specific information.

repl> repl.inspect(repl)
[...]
<object>.back=[function]
<object>.sayHello=[function]
    just say hello
<object>.module=[object]
<object>.util=[object]

<object>.setenv=[function]
    Takes a name and a value and stores them so that they can be later ...
<object>.getenv=[function]
    Given a name, returns a value previously stored via setenv().

<object>.pushenv=[function]
    Takes one or more names of values previously stored via setenv(), a...
<object>.popenv=[function]
    Takes one or more names of values previously pushed via popenv() an...
<object>.print=[function]
    Converts an object to a string and prints the string. Appends a new...
[...]
repl> repl.doc(repl.print)
TYPE: function
NAME: print
ARGS: data, appendNewline

Converts an object to a string and prints the string. Appends a
newline unless false is given as second parameter.

repl> 

When you're done:

repl> repl.quit()