I was looking for an equivalent Firefox extension to Chrome's "Window Resizer" that I use for web development. After some search, this one seems the best alternative. It works very good.
Feature suggestion: Custom default presets for common monitor and mobile resolutions would be a great addition and would make your extension stand out from the crowd.

Works exactly as described, with the ability to make custom resolutions, reorder them as you wish, and with two clicks you can resize your window to your preference!

Wish I could give more than 5 stars. Well done.

My evaluation is 5 if I can save the current window size.

January 2, 2018 Addendum:
Since it became possible to set the current window size at Ver 0.6.1, I changed it from star 4 to star 5.

Not so nice that the Hotkeys are fixed to Alt+x - because the same Hotkeys also switch between open tabs ...

simple and useful

Very nice & useful add-on! Thank you

I've created two window sizes that provide the best aspect ratio for my most used applications, a news paper and weather site. Everything worked as expected. It would be nice to have an edit feature. Currently the add-on can only Add and Remove. Looking forward to improvement(s) in this app.

Increased to 5 star since the app now gives "edit" capability. Also, there is a new feature that allows you to resize the window and select, name and save the current size. Nice feature! Thanks for the good work.

Potentially useful, but buggy. I was only able to add one size, 800x600. When I then tried adding 720x1280, the add-on wouldn't let me save the new size

This is exactly what I was looking for today. Perfect timing. Glad it doesn't require the window position parameter. Thank you for this useful extension.

Rather pointless without 'window position' property.

UPD. I see. Sure you have your reasons.