I've tested it myself have have found that having Adsense on a page has no effect on rankings. However, if you add Adsense code on your page and then take the code off
or don't have the code on there and then add the code on then you've made a change to your page. And Google seems to like that. So, in that sense it could potentially have an effect on rankings.
As for
SEO firms that have Adsense on their site, you have to question a company's motives and overall objectives if they have
any forms of advertising on their website, not just Adsense.