You can't find firmware because there is none, as far as I know anyway. Your drive already has the latest/greatest/bestest/beall/doall firmware!
In contrast to SandForce based SSDs, Intel has no need to update it's firmware constantly, or even occasionally, only rarely.
The latest firmware for the 510 is PPG4. AS SSD Benchmark displays the firmware in the upper left portion of it's screen. The Intel SSD Toolbox will show the firmware version in the View Drive Information option, scroll down a bit. The IRST interface in Windows will show the firmware version. Check the label on the box for PPG2 or PPG4.