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By default, Firefox cannot be resized to a window width smaller that about 400-500 pixels. However, you can override this restriction by customizing Firefox with userChrome.css

To resize Firefox windows smaller than the enforced minimum width

1. create or edit a file named userChrome.css in a chrome folder inside your Firefox profile directory. (on Windows, you can go to this folder "%APPDATA\Mozilla\Firefox\Profiles\")
2. Add custom CSS rules in that file to override the minimum width restriction
3. In Firefox, enable the loading of userChrome.css by setting toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config.
4. Restart Firefox and then you can resize the window to less than the enforced minimum width