I was looking at about:config with the filter useragent and found the variable general.useragent.vendor which has a string of Firefox/3.5 Maemo Browser 1.7.4.8 RX-51 N900.

Tried to change it to something else but community.letsencrypt.org still complains about an old browser.

Also the new string inserted doesn't seem to be saved after a browser restart (I find the old string instead).

Could community.letsencrypt.org be using this variable to identify the device?