I was having this issue too I deleted the Firefox app off my phone. Took it off default put Safari back on. The problem was the app had not taken all of my information when I downloaded the first time and I have 17 Pro. So I’ve downloaded it again and I put in email & password and it went right to Firefox but it’s still a Google search engine unless I choose to change that. Also, instead of doing a QR code go to my computer. I did email. I’m not sure if that had an effect or no. Oh, and yes, I had to change the default to Firefox from Safari, but it seems to be working good now. I was wondering if there isn’t a better search engine for me to use other than Google?

Is the issue still happening? I do not understand why a Google account login should block using Firefox on iOS after installing, so I'd like to investigate a little bit. Would you be able to attach screen shots or recordings of when it happens? It would help understanding in what stage that's happening. Thanks!