Firefox trapped in adware scam loop

My Firefox on iPhone has a malware that pops up a box urging me to call a phone number (which I won’t do) about a payment. It has Cancel and Ok buttons, both of which trigger another box from the dialer containing the same phone number. I cannot get to the FF settings or escape from that page because the pop ups immediately come back and reload the page. The site looks like appleinstantsupportpays[.]z13[.]web[.]core[.]windows[.]net. All of the online advice relates to FF on Windows or Android. How do I clear the malware from my phone?