There are several ways to tether your Android mobile phone, but most require root access to your phone. This is a simple and unobtrusive way to tether, and should work with Linux, Mac and Windows.
Install the Proxoid Android application for the marketplace. Below is a QR code for Proxoid
Download and extract the latest Linux Android SDK from http://developer.android.com/sdk/
Once downloaded extract the archive
tar -xvzf android-sdk-linux_x86-*.tgz
Enable USB debugging on your Phone via Settings > Applications > Development
Connect your Android phone via USB, start the Proxoid application and tick the start box
Forward the ports on your Ubuntu PC
android-sdk-linux_x86-*/tools/adb forward tcp:8080 tcp:8080
Setup your desktop environment to use the new proxy. In Gnome you can setup a New Location in System > Preferences > Network Proxy
Make sure Firefox and any other application you wish to use is using the system wide gnome proxy settings
To disable all you need to do is switch back the proxy settings
