Wanted to make counter-argument, opened new tab, found you are right. Google Chrome mobile on the other hand gives me both sides.

I have no idea if they have a bias on what they send to Firefox, but their approach (send stories with a list of other sites that signal you might be interested) is obviously going to facilitate a filter bubble. Is your usual filter bubble right wing? If it is that suggests a bias on their part, otherwise maybe they're just amplifying your left wing browser history?