It's the only browser I have installed besides Safari, and my default browser but instead of just opening the link with my default browser, it advertises these other browsers to me and makes me click 'Default browser app' by default. wtf I'll be turning that off, should have never been a feature.
I hate Apple as much as the next guy but this is some next level complaining.
Step 1: Select "Open with Default."
Step 2: Accept that the mere suggestion that you use literally any option that's not your preference isn't a personal attack and the end of the FOSS world.
Click "Default Browser App" on this screen. That will open the link in Firefox if it is configured as your default app. I have the same issue using Brave.
Blame Apple. This is from when Safari was forced to be the default in all cases, so Google hard coded their other browsers to open the link. The possibility to set another browser as default is pretty new, probably Google is waiting to switch to the new API or otherwise people with iPhone 7 or older wouldn't be able to run the app anymore (no more updates for them)