Configure your apache publish directory for ClickOnce applications
At the root directory of where you are publishing your ClickOnce application, create / edit a .htaccess file. (click here for the apache documentation on AddType) (if you still need help with apache I suggest the following book Apache: The Definitive Guide (3rd Edition) )
Add the following lines to it:
AddType application/x-ms-application application
AddType application/x-ms-manifest manifest
AddType application/octet-stream deploy
For some reason I also needed the following line in my .htaccess file
AddType application/x-msdownload dll
For more reading on ClickOnce check out Smart Client Deployment with ClickOnce(TM): Deploying Windows Forms Applications with ClickOnce(TM) (Microsoft .NET Development Series).
Use FTP when publishing from Visual Studio, Unless you want to install FrontPage Server Extensions on your Apache, Yuk!
Using a ClickOnce application with FireFox
Currently ClickOnce applications will only work with Internet Explorer, you CAN however use FireFox, but you will need to install the FFClickOnce extension.