It appears that it’s impossible to upload texts in Firefox. The upload button never appears. I have to open Chrome to upload things, which is a little bit annoying.
I’ve just tested the upload page on Firefox 145.0 on macOS, using pasted text, and uploading .epub, .txt and .mp3 files, and it works fine for me.
Please check that you don’t have any browser extensions installed which might interfere with the Readlang website.
If it still doesn’t work for you, please provide some more details so I can try to reproduce the problem.