Flexible means that it can push other elements (buttons) to the right or left if placed at the far right of a toolbar. So it doesn't have a specific width. Flexible space (1) doesn't work on a toolbar that has other element with a larger flex setting like the location bar (400) or the search bar (100). You would have to use code in userChrome.css to set a specific left or right margin to a button to get a special layout.

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

still doesn't make sense to me. this is what i've done...first of all, i installed FF4 and didn't like it. but i did play around with some of the toolbar settings to try to get rid of the new "add-on bar" at the bottom. so i went back to FF3.6 and continued to play with the toolbar settings. i put the google search in the menu bar, but wanted to space it to the left, off the right margin. that's when i discovered the "flex space". you can indeed put the flex space in the menu bar to shift an item (button OR google search box) to the left or right. where i run into an issue is, in order to shift an item from, let's say the far right margin to the center, i would have to insert multiple flex spaces or i could also use the other "space". although, the "space" is two or three times as wide as the "flexible space". maybe the trouble that i'm having is the name"flexible space" which seems to imply that item has a flexible width. both "flexible space" and "space" do exactly the same thing, except "space" is wider than "flexible space". am i right? did i miss something? here is a screenshot to illustrate.