Did you even try looking? Settings > Customize > Toolbar. The settings list is not long on Fx mobile and the ability to change the position has been there since the day of the UI refresh.
The question was not about bottom vs top, iiuc. It was about having the tabs always visible vs behind a tab button. As far as I can see, there is no way of doing that.
Not that I would want it, myself, since my current counter is 48 tabs and I don't want a bunch more hit regions when I'm thumb-browsing.
It appears I did miss a part of the question. My tab count is ∞ because I'm over 99 and tabs are unloaded. At that rate, if I were on a tablet or something, I'd prefer access to TreeStyleTabs or the like anyhow because horizontal becomes a squished mess. I assume the ctrl+tab as well as alt+{1-9} shortcuts still work.
Not that I would want it, myself, since my current counter is 48 tabs and I don't want a bunch more hit regions when I'm thumb-browsing.