Where are these conversations happening? I could see a lot of enterprise-focused groups potentially getting behind OnlyOffice, but individual home users? Not so much.
EDIT: My mistake! I didn’t realize that there are standalone versions of OnlyOffice in addition to the web app version.
When I go to that URL on a stock, direct FF install, I still see that notice.
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.
I apologize, because between OP’s post and looking at the OnlyOffice website, I got the impression that it was only a web app, requiring a web server to run. After reading another comment here I looked harder on the website and found the download links for the standalone versions.