Turns out not to be an issue after all, since once I chose the books via library > file tree > mnt blah blah blah, fbreader remembered that directory, unlike the fbreader version I had been using where I had to go and choose it each time until I actually typed in the correct directory in settings. I thought I had to do here and couldn't, which is why I posted, so all is now well since fbreader now adds whatever directory you chose via file tree to its library. But everyone probably knew that since everyone was probably updating their fbreaders all along except for me
