That is caused by this CSS rule for the main window.

• chrome://browser/content/browser.css

#main-window:not([chromehidden~="toolbar"]) {
  min-width: 390px;
}

You can use an overriding rule in the userChrome.css file with a lower min-width.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#main-window:not([chromehidden~="toolbar"]) {
  min-width: 200px !important;
}

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

Hi Cor-el,

Thanks for your reply. That didn't seem to work.

My profile didn't have the Chrome folder in there. I create one, and put the CSS file in there, reset FF... but nothing.

For the time being, I can use tools > web developer > responsive design view but it's not as intuitive as actually resizing the browser window using dev tools.

I notice that Chrome has the same bug / feature. Do FF26 and Chrome run off the same settings?

See if this addon works https://addons.mozilla.org/en-US/firefox/addon/window-resizer/

"My profile didn't have the Chrome folder in there."

The folder name chrome needs to have a lower-case c - Firefox won't recognize Chrome (with an upper-case C) as a valid folder.

Also make sure that you didn't save the userChrome.css file as a text file because that will Windows make to add a hidden .txt file extension (userChrome.css.txt).

You can see the real file extension (type) if you open the Properties of the file via the right-click context menu.

The add-on did not work either. So far setting it to Responsive has worked the best for me.

toddy victor on December 20 said, "See if this addon works https://addons.mozilla.org/en-US/firefox/addon/window-resizer/"

This Add-on says, in red: "Not available for Firefox 26.0."

I had three windows set up next to each other with no problems for about a year. They would retain their size and position with every restart, etc. With the 26.0 update, I can no longer use them together because they are now all too large. I hope that this is a bug and will be fixed without my having to jump through a lot of hoops to work around the problem, since I am NOT a techie!