It's difficult to create new window sizes. Sometimes it creates two instead of one, and other times it just deletes them. Plus, even after removing the default ones, they keep reappearing automatically.

Doesn't work. The instructions say "simply open toolbar popup UI and click on desired layout"... er, what toolbar popup? I don't see any new icon or any UI for this extension. Maybe it worked in the past, but it doesn't work now.

プリセットを消しても復活するので、自分の用途には合わなかった

blocked me?

perhaps reading the reviews might have helped, i was thinking this would allow me to set the size of a page, something i was looking to use for twitch and youtube to scale the videos so they'd look good at low resolutions on my giant monitor, without making the browser also small.

this browser does what it says tho, it resizes the window and not the page. unfortunately not what i wanted and i didn't understand what it was saying until i tried it.

It doesn't work okay in my FF 70.0.1 64-bit. Deleted.

Hard to create new window sizes. Some times it creates two and some time it just delete. Plus, after deleting the defaults, they keeps coming back automatically. I mean what type of addon is this one?

This is helpful to spoof websites into showing them a specific screen resolution. But keep in mind that your URL bar and possibly bookmark bar reduce the size of your inner Firefox window detected by websites. So you will have to create a custom preset and add the number of pixels to your desired size.