How can I change some of the system default preferences used for the guest and new user accounts?
First of all you may need to unhide the hiden files so open terminal and run those commands :

defaults write com.apple.finder AppleShowAllFiles TRUE

killall Finder


To switch back, do the same but substitute FALSE for TRUE. 

Now 
Login to your guest account, there set chrome as your default browser (or even Firefox) 
Then do not logout ! This would erase your session, so use the  on the top right corner use the swich user drop down menue, 
Now that you are back again in your Admin account,
Find the new temporary guest home folder, under  

 Users/Guest


Select all the folders inside of it,
And copy them in : 

 /System/Library/User Template/English.lproj/    


English or whatever your languish is if you are using your computer in french this should be :

  /System/Library/User Template/French.lproj/    


as you can see in this folder there was the defaults folders for the guest account 
By modifying them you have just modified default appearance of the guest user account
You may want to save this folder in a safe place before, in case you want to undo your work.

For reference , 
You just have to follow the steps as shown in this video : 

https://www.youtube.com/watch?v=JU-W2-_c5bA

Or if you prefer text version :

http://www.michaelsmac.com/modules.php?name=News&amp;file=article&amp;sid=394