Why don't|you set it to open new tabs with a blank page? I have that as my setting and I forgot there were even ads on the start page.

Firefox does have ads on by default for the startpage, but they can be turned off in the settings in a fairly easy and obvious way. So even as obnoxious and demoralising ads, I don't think it is fair to say these ones are "persistent and unavoidable". (It's not easy to remove the Firefox logo from the new tab page, and that annoys me, but I wouldn't call it an ad.)