If the code isn't working then try to add the !important flag. The negative margin is to move the scroll bar out of view, so it is still there, but only not visible. That is to prevent scrolling issues.

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

#content browser {
 margin-right: -14px !important;
 overflow-y: scroll !important;
 overflow-x: hidden !important;
}

Does other code in the userChrome.css file work?

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userChrome.css file in the editor window
• Make sure that the userChrome.css file starts with the default @namespace line
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css.
  Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file

I tried to use the !important flag but the overflow-x and overflow-y commands didn't work. It seems that only the margin-right command still working. But it's ok, using the nenegative margin like you told me, I could move the scroll bar out of view. Thanks a lot.

Now I'm having another problem, I'd like to hide the webpage until it's all loaded or to use a fade effect to hide the page for sometime (5 seconds for example) or make the content transparent for sometime, do you have any idea how I could do that?

Thanks!