I tried version 1.6.1.0. It's very good for an early version, as fast as Sony's for me and stable enough. Good job!
If you add the following (in no particular order), I'd replace Sony's firmware for your own as my first option:
1. Full history, not just a list with the latest ebook(s). This means history as in the last 10 pages at least. Very useful when you somehow lose your page after a mispress or after letting a friend try your ereader. Also very useful if you follow links (when enabled).
2. Enabling links (at least in ePUB and PDF), with navigation UP/DOWN or LEFT/RIGHT according to the user settings. I haven't tested CHM, but I hope links work there, otherwise there's no point in using CHM.
3. Set the default Books folder to be the folder where books are copied by Calibre (and I'm guessing Sony software), in the same format (author\title). If not set by default, at least allow to navigate there easily.
4. Public release of any open source code you have used / remixed. I have no problem in supporting commercial projects over open source ones (specially if those open source ones do not support my PRS-300), but it's not cool to "steal" (not offering attribution) from open projects.
If you add the following, I'd ditch the Sony firmware completely (consider this a wishlist):
1. HTML rendering with full CSS. I'm not sure how CSS(2/3???) compatibility is these days, but let's say as good as Firefox / Chrome.
2. Dictionary support, hopefully in Stardict format, hopefully using the right-hand buttons as T9 input or similar.
If you need help with Spanish translation, just let me know. The Spanish in your firmware is decent sometimes, but sometimes it was terrible, too. Thanks for giving the 18n resources in editable format, so users can tweak them at leasure.