I don’t think it’s browser specific or a direct targeting of Firefox; that’s likely confirmation bias. I see rolling captchas and the annoying ones that have a delayed fading in and out even on Chromium forks. I think the biggest reason for seeing them is VPN usage. When I turn off my VPN, I either don’t see any captchas or it just automatically shows the green check when I start them.

There’s a FOSS fork of Signal which removes Google dependencies from the software.

Signal-FOSS

A fork of Signal for Android with proprietary Google binary blobs removed. Uses OpenStreetMap for maps and a websocket server connection, instead of Google Maps and Firebase Cloud Messaging.

https://github.com/tw-hx/Signal-Android