Despite turning off settings to open new tabs, every website I open browsing on mobile opens a new tab

That shouldn't happen, I've just set Homepage > Opening screen to last tab and when I open firefox it defaults to the last tab that I was on before exiting the browser

I had to adjust a ton of settings to stop it from downloading in the background every file I just wanted to view or print

Iirc its just setting browser.download.open_pdf_attachments_inline to true at about:config