If you use Window Resizer Beta (or the like) on Chromium-based browsers, this is the Firefox analog you're looking for. In fact, @FullStackChoreman's add-on has more functionality. Written by a developer for develolpers. Excellent work.

This is a simple and intuitive extension allowing you to set as many preferred window sizes as you like, and easily switch between them. The menu also shows the current window size, which can be useful for web developers.

(There are a number of other window resizing extensions, but most of them have very strange interfaces. This one works in a sensible manner and works well.)

This is what I wanted.

The best add-on for resizing Firefox, but there is still room for improvement. For example there should be option to change default hotkey or center window after resize. Also there is a bug in Mac OS, after changing position of my presets Firefox add-on popup closes.

Great work, I really appreciate that

Allows custom sizes with names/notes, no bugs (unlike the other 4 plugins like this I tried before)