Technically, both Chrome and Firefox are adware too, since Google's main business is ads, and Firefox/Mozilla get a lot of money from Google to display Google as a search engine in Firefox (an ad :) )

Firefox doesn't sell BATs, in-browser notification ads, or new tab takeovers. The closest you can get is a pinned site in the new tab page (new installs only) and ads in Pocket, or whatever they're calling that new tab thing these days.

https://www.mozilla.org/en-US/advertising/solutions/

https://brave.com/brave-ads/browser/

Calling Firefox adware is a stretch at best, and disingenuous at worst. Adware doesn't mean that the software survives because of one advertisement that that user can turn off.