Don't expect Firefox for Android to get tablet UI anytime soon
www.androidauthority.com
Mozilla has confirmed that optimized tablet UI is not a priority for the Firefox on Android team and that it is focused on other matters.

TL;DR (From Android Authority)
• Mozilla responded to a review mentioning the lack of tablet UI optimization for Firefox on Android.
• Mozilla confirmed that its team is not working on optimizing the tablet experience at the moment.
• Mozilla did, however, say that it hopes to revisit tablet optimization when time and resources permit.