Please explain what you are saying. safely eject my flash drives  ? ! ?

Are you saving any files to these Flash drives when downloading/saving file from within Firefox?

It is also possible that your security (Anti-Virus) software is causing this problem.

I agree that something must be keeping the usb flash drives in use and not letting you unmout it. Say maybe a download has not finished writing to it yet?

FredMcD said

Please explain what you are saying. safely eject my flash drives  ? ! ?

You know, those usb Flash drives (aka jump drives, aka pen drives etc) that has existed for some fifteen years now. The message is basically saying something is still making use of drive and cannot be safely unplugged from usb until then.

If you use Firefox to download anything to that USB device, Firefox will "watch" that drive. You should clear all those downloads from the Download Manager or Library > Downloads (via the context menu in each place) before trying to eject / unmount that drive.