User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:77.0) Gecko/20100101 Firefox/77.0

Steps to reproduce:

Take the Mozilla Developer Edition
open a site
right-click "Inspect Element" (o)
Responsive Design Mode (ctrl + Shift + M)
refresh the Site with (Ctrl + R)
now try remove Responsive design mode (ctrl + Shift + M)

if you do not refresh the site it works well.

Actual results:

you wil not be able to remove the Responsive design mode (ctrl + Shift + M)

Expected results:

you should be able to remove Responsive design mode (ctrl + Shift + M)

ir is somehow similar to Bug 1535552

Thank you for filing! This issue might be the same as Bug 1636634.

Would you be able to paste what's reported in the Browser Console?

Christian, see comment 2, can you assist in tracking this down?

(This doesn't reproduce for me in macOS or win10, so it may have been fixed by Bug 1636634)

S1 or S2 bugs need an assignee - could you find someone for this bug?

I'm also unable to reproduce this issue on Firefox Developer Edition 78.0b2 (64-bit) on both macOS and Win10. It's likely this was a duplicate of Bug 1636634. Perhaps let's wait on the reporter to confirm whether or not they can reproduce the issue still.

I'll resolve this issue as a duplicate of Bug 1636634. If anyone else is able to reproduce this, please feel free to reopen!