a) This is completely dependent on your browser settings. For example in Firefox/Iceweasel: Edit, Preferences, General, Downloads, Save Files To.
b) ASSUMING the .deb is from a source that you completely trust, and that the .deb is tested to work with your specific Debian release:
Code:
# dpkg -i /home/snowpine/Downloads/teamviewer.deb
(The # symbol indicates root terminal, use 'su' or 'sudo' depending on your security settings; and replace the path with the actual path to your download.)
If you have the application
gdebi installed then you can just click the icon in your file manager, easy!
Code:
# apt-get install gdebi