Go ahead with installation of the new firmware. Get the web interface up on a browser, and go to the Admininstation-->Firmware Upgrade menu. Select the filename of your new firmware, and click on the upgrade button. Then, during the minute that it takes to complete, grab a piece of scrap paper and a pen, and look in the upper right corner of that page... and write down the version of the existing firmware. Don't make a mistake, because it will be gone in about 1 minute 10 seconds or so... ;-)
Or just look at the upper right corner of almost any of the web pages. Most of them should have it.