If this is useful for anyone: those who use Diaspora* had a known issue with not being redirected to mobile version in mobile Firefox.

As it happens, there are explicit mobile URLs:

https://joindiaspora.com/stream.mobile
https://joindiaspora.com/activity.mobile
https://joindiaspora.com/notifications.mobile

(Or change it to your pod from joindiaspora if needed).

Logging in didn't work in the mobile version though (so you'll need use the regular one at least for the log in step).