Firefox has been able to run on RISC-V for as long as I can remember. I'm pretty sure I remember SiFive doing a demo back in 2018 at FOSDEM which included the browser. However generally GUI environments are still quite slow, so it won't be very usable.

Edit: This one I think: https://archive.fosdem.org/2018/schedule/event/riscv/ but I can only see them running Quake, not Firefox.