Yep, it’s probably easier to get an Android device and install readers on it than to try for a prepackaged FOSS reader.
I use several apps on my Android phone, but mostly Kindle (for Kindle, duh), PDF Reader (for PDFs, duh again), and Lithium (mostly for EPUB but pretty much everything else, too). I get most of my e-books as DRM-free EPUBs and PDFs.
When I go to that URL on a stock, direct FF install, I still see that notice.