How do I show the available versions of a package in Ubuntnu?

There are several methods, using the command line.

1. If you don't know the exact name of the package, you can use dpkg -l which lists all the installed packages, and pipe it through grep to isolate just what you want. Example: dpkg -l|grep firefox. This is what I use most often. Pro: helps you find stuff easily. Con: may get your more than one package (if there are several related packages with common bits in their name). But I still prefer this approach.

2. If you know the name of the package, you can use dpkg -s firefox. Downside: you have to know the exact name of the package, which is not always straightforward. On the upside though, this command is more powerful; if the package is not installed, it will tell you so; if it is installed, it will give you lots of info, more than just version (dependencies, description, etc.)

I don't know how to do it in apt, but both Synaptic and aptitude (the ncurses interface) have a menu that shows all available versions of a package. In Synaptic there's a "show all available versions" menu entry. In Aptitude, simply view the package.