1. Open Scratchpad either by pressing Shift+F4 or clicking the Firefox button, opening the Web Developer submenu, then choosing Scratchpad.
2. In the Scratchpad window, type window.resizeTo(1275,700);
3. Press Ctrl+R or open the Execute menu and choose Run.

This will only work if there is an XUL (chrome) window like the about:newtab page is open and selected. Such pages have elevated permissions and allow scripts to change the window dimensions.

Hi Gingerbread_Man, thanks for your response. I tried that and it didn't work. No effect. I have JavaScript enabled (in Tools, Options), and Allow scripts to resize popup windows is enabled. Combining your reply with cor-el's, I'm able to do it. Thanks!

Hi cor-el. Although I don't know what you mean by a XUL window, I tried the script on a new tab and it worked. So I guess it is a matter of permissions. I guess I could open a new tab, run the script, then switch back to the original tab. Thanks!

It also works with the "Tools > Add-ons" page (about:addons), so that is an alternative if the new tab page is set to about:blank (like I prefer).

It doesn't matter if Tools > Options > Content : JavaScript > Advanced > Allow Scripts to: "Move or resize existing windows" is disabled.

Alternatively, you can install an extension. For example, “Web Developer” allows you to resize the window from its toolbar. It works regardless of which page is open, and it's simpler than typing Javascript code in a separate window.

• https://addons.mozilla.org/firefox/addon/web-developer/