While in Coda, I press Command + R to refresh Firefox, saving one (measly) click.  There are probably better ways to do this like using the LiveReload plugin. 

First, create the Service in Automator:

1. Open Automator
2. Select Service
3. Click Utilities
4. In the Service receives select no input (1st dropdown) in Coda (2nd dropdown)
5. Click Run AppleScript then Paste the code below in the AppleScript window
6. Save the service and name it Refresh Firefox in Coda

tell application "Firefox" to activate
tell application "System Events"
	keystroke "r" using {command down}
end tell

Second, setup the keyboard shortcut:

1. Click System Preferences
2. Click Keyboard then Keyboard Shortcuts
3. Select Services
4. Check off Refresh Firefox in Coda
5. Click Application Shortcuts
6. Add a new Shortcut and enter in:
7. Application: Coda.app
8. Menu Title: Refresh Firefox in Coda
9. Keyboard Shortcut: enter your shortcut

Restart Coda and press Command + R to refresh Firefox.