Apple - Open a specifc URL in Fluid.app

I had the same problem and wrote a little AppleScript to handle this. You can find the app here: https://github.com/biafra23/Default-Fluids

You need to make this script your default browser. This can be achieved with Safari: Menu -> Safari -> Preferences -> General -> Default web browser

The source is in https://github.com/biafra23/Default-Fluids/blob/master/default_browser.app/Contents/Resources/Scripts/main.scpt

It contains sections like this:

if this_URL contains "facebook.com" then
  tell application "/Applications/Fluids/facebook.app"
    activate
    open location this_URL
  end tell
else if 
...
else
  -- default browser here
  tell application "/Applications/Firefox.app"
  activate
    open location this_URL
  end tell
end if

You need to make changes to those according to your setup with the AppleScript Editor.

It is not as convenient as choosyosx but its free.

Developer of Fluid here. The only way I know of to do this is with an additional utility like Choosy or Browser Fairy (app store link).