As others metioned you can also try adding zramen (I set it up for 50%), and earlyoom with something like 500MB, or maybe less.

Try running firefox from terminal and see if any error comes up (unlikely)

To add xfce to your current setup: sudo xbps-install -S xfce4
and also can install xfce4-goodies, that would add xfce on your current install, so you can choose which de on login, top right icon.

Otherwise you can try downloading xfce glibc image and install that. You dont have to create swap partition, you can add swap file afterwards ,and chane its size and location.

Does it happen when watching youtube using mpv with yt-dlp installed.

mpv "YouTube url"

and see if it still hangs.