How did you configure Firefox to point at Fiddler in the first place?
If you set Firefox to use Fiddler by configuring its proxy settings to "Use the system proxy" then this works automatically.
If you set Firefox to use Fiddler by using the FiddlerHook plugin for Firefox, you should use the "Use Fiddler Automatically" mode.
If you set Firefox to use Fiddler by manually pointing its proxy settings directly at 127.0.0.1:8888, then you have to reverse that procedure manually as well.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.