User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36

Steps to reproduce:

Running Firefox Developer Edition 69.0b2 (64bit)

1. Open Firefox via any means
2. Visit any website
3. Go into Responsive Design Mode (Ctrl-Shift-M for PC)
4. Go to a different page via hyperlink (i.e. not typing in a new URL in the address bar)

Actual results:

New page loads as usual, but new URL is not reflected in the address bar; still shows URL of first website.

Expected results:

Address bar shows URL of new page after navigation click

Facing same issue on
Firefox 69.0a1 Nightly and just verified on Nightly 70.0a1 as well
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:69.0) Gecko/20100101 Firefox/69.0

one more thing i noticed is when we go from responsive mode to normal mode ( responsive => normal )
address-bar becomes empty have attached a gif

Hello, thank you for filing this issue. I am able to replicate the issue you described. We will make sure to get this issue on our list.