Yes and No. The 'Kindle Fire' Mobi is your output profile. "the output profile tells the conversion system how to optimize the created document for the specified device. In some cases, an output profile is required to produce documents that will work on a device"
This includes options like screen size(for image resizing), margins, font size, and options like that.
If you'd prefer a default option, when you go to convert you can choose the input and output profiles on the 'Page Setup' option when converting ebooks using the Calibre GUI. There is a
'Default Input Profile' and 'Default Output Profile' you can select.
For more on Input and Output profiles: http://manual.calibre-ebook.com/conversion.html#page-setup
To prepare it for the Kindle without connecting, right click on the ebook in your Library window. In the 'Convert Books' menu, select 'Convert Individually" or "Bulk Convert". To select the output profile(Where you will choose your device), go to the 'Page Setup' screen of the configuration window that pops up.