With an old phone Firefox is indeed slow. Startup is slow, page loading is slow, even scrolling is sluggish. On Chromium browser the performance is a lot better if your phone is not that fast.
On Firefox it works if you change the settings, but unfortunately i don't know any Chromium browser with that feature.
In settings there is "open links in apps". If you change this links are indeed open in the default browser
Is it necessary to click on the ad? Or is it enough to go to the web page where the ad is displayed?