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Edited : as mind was faster than fingers.

Am not comfortable with what I see for code changes, seems for everything older. Can have a look. https://www.google.ca/search?q=Firefox+hide+scroll+bars&ie=utf-8&oe=utf-8&gws_rd=cr&dcr=0&ei=uKP_WfW4GYaojwPf44SQAw

You can try if this code in userChrome.css works for you to move the scroll bar out of view. You may have to adjust the negative margin values.

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

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

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

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

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

• 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

cor-el said

You can try if this code in userChrome.css works for you to move the scroll bar out of view. You may have to adjust the negative margin values. Add code to the userChrome.css file below the default @namespace line.

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

---

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

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

---

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

• 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

Edit : Actually while this method does hide the scrollbars, it introduces a bigger problem. Content is cropped to the right and bottom. For example, if an image would normally fit your browser's height but not width, and you have adjusted the bottom margin by -15, the image would be vertically cropped 15 pixels.

Images are cropped at the bottom and to the right, when the horizontal / vertical scrollbar respectively would not be used. The only time the browser displays the whole image, is when both scrollbars would be shown if they were not hidden.

This perhaps is tricky to understand, but do a test :

Find a vertical (portrait) image that only fits the height of your browser, save it, open it with a separate image viewer, and compare the two.