You're not alone, I've been using Firefox on Android for two years now, and I've had this problem from the beginning. The first time I launch Firefox and load a page it stops a quarter of the progression bar for ten seconds, and then loads fine. Once pas that everything works perfectly. It's very annoying and I don't know why this happens.
https://support.mozilla.org/en-US/questions/1217722
Change it to "Vertical", if that's what you mean.