I think people are getting confused because Flashtool itself outputs a message saying that USB debugging should be turned on. That is required for some other functions of Flashtool. You 100% do not need USB debugging enabled to flash an FTF. The device isn't even booted into a state where that could possibly affect anything when in flashmode.
As for drivers, try using this package - Flashtool and fastboot drivers.
If you're using Windows 8, there are additional steps required to install the drivers that are linked to in the OP of that thread.
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