So I noticed an article mentioning you couldn't get clickonce working in Opera or Firefox. Well, I don't know about Firefox but I've had it working in Opera 8 for some time now.
The first problem you hit is that the framework isn't returned in the user agent. To solve this create a folder in your my documents called OperaJS and a new text document adddotnet2.js. Include the following:
// Adds the user .NET CLR user agent
navigator.userAgent = navigator.userAgent+'; .NET CLR 2.0.50727';
Now when looking at a clickonce install site you get the option to just install (make sure you actually have 2.0.50727).
The next step is to make Opera pass the url to the .application file format to EXPLORER. I would pass this to the appropriate file to run it, but then it won't be in the right context so we'll settle for a little dodgyness.
Click Tools, Preferences, Advanced, Downloads then add a new mime type of application/x-ms-application with an extension of application, then check "Open with other application", enter "explorer.exe" and check "Pass web address directly to application".
Now you should be able to navigate to clickonce applications and use them as if you were in IE!
Update 5th July 2008:
There is a firefox add-in here.Permalink