The same holds true for that Seamonkey browser you were looking for.  When you're trying to get software that is 
not in the repositories you literally have to 
add another repository (called a PPA for Personal Package Archive), which is why we need the "command line" (terminal).
Unlike Opera (no offense, [member=6960]Vera[/member] ), Seamonkey 
is fully open-source, free software and it will be completely familiar to you, as a Firefox/Thunderbird user):
Open the terminal and type this:
 and enter your root password.
This opens the File Manager in "super user" (Administrator, or in Linux, 'Root') mode.  You 
have to be root in order to edit the software sources file.
Navigate to 
etc/apt/sources and find the file named 
sources.list.  Right-click on the file and open it with Text Editor.
Add this line to the the bottom of the sources.list file:
Quote:[pre]deb http://downloads.sourceforge.net/project...ozilla/apt all main[/pre]
[pre]
Save and close the file. Then run the following command to import Ubuntuzilla public key to your keyring so that 
the integrity of packages downloaded from this repository can be verified by APT.
In the terminal type:
[/pre]
Code:
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 2667CA5C
And then 
(wait for it to update, and when it's finished, 
Code:
sudo apt-get install seamonkey-mozilla-build
[pre]
This will not only install Seamonkey for you, but will also add it's PPA to your software sources 
so that it will be included in future updates.
Opera probably has a PPA too, but be careful about adding a whole bunch of PPAs to Linux Lite.
I really hope this helps.
[/pre]