When I was younger, paperback because they were cheap. These days I prefer hard back because the font is easier on my eyes.
That I said, like everyone else it seems, I do much more reading on my Kobo. It has the font benefits if I need it, but huge space savings. I still have a large collection of books but every time I move I tend to move more and move of them into ebooks.
Mozilla org isn’t the concern. Mozilla Corp, the for profit company, makes Firefox and has to worry about things like revenue for the most part. Mozilla org used to develop it and could fold it back in if it went really bad, it would definitely hamper development but being the premier browser is more of a Corp goal than an org one. Most likely the corp will just find a different search partner again (Google hasn’t always been default).