Okay so Manjaro is a brilliant fork of Arch, although not as bleeding edge as Arch base, its stable and works well, but there is some things you have to do after install.
1 - Update your system
Your system will be slightly out of date, so pop open a terminal and get the update command running
sudo pacman -Syyu
You may need to do this twice, I do on my system, but it will only take a matter on mins on a broadband connection.
Personally here, I would reboot just so things settle a thing, you dont need to but its just something to do.
2 - Update the kernel
I find updating the kernel in manjaro easier than base Arch, although both of them are not hard, Manjaro helped to make it easier for new people, Paste command into your terminal.
sudo mhwd-kernel -i linux313 rmc (Use rmc if you wish to remove the current 3.10 kernel)
This should also add the headers for stuff like video drivers, this has to be done manually in Arch with Catalyst.
Again reboot, I would say this is the time you should reboot, after all you have just performed a kernel update.
3 - Get Steam running
Okay so we love to play games, and steam is pre-installed for us to use :) so go ahead, open it up, sign it and get downloading.
4 - Get ClamAV
Now this is not needed, but if like me you have Windows systems on your network, or you use USBs that connect to Windows/OSX machines, its advised as Linux has a mounting addiction.
sudo pacman -S clamav
Then enable it
sudo systemctl enable clamd.service
Then update it
freshclam
Let it update
If you want the ClamTK (GUI Edition) use yaourt -S clamtk and follow instructions, never use root for building packages with the AUR or yaourt.
5 - Get firefox sorted.
Okay so for Firefox you need some additional addons to make it shine, this are security related.
Bluhell Firewall - WOT - Noscript
Bluhell - Adblocker - Lighter and better than Adblock
Web of Trust - Site ratings based on other users experiences.
Noscript - Disables java and only allows it to be run on users permission.
And there she goes, Manjaro ready in a few easy steps, if there is anything else you add to your setup, add it in the comments and I shall update the list.