You can use a custom style rule to do this, if there aren't any nice themes that have this feature.

Try this. Open:

Help > Troubleshooting Information > "Open Containing Folder"

Your active Firefox profile folder will be displayed. Double-click the chrome sub-folder.

Do you have a filed named userChrome.css ? If not, right-click, choose New > Text Document and change the name to userChrome.css (note that the name must end with css and not TXT).

Open the userChrome.css file in a text editor such as Notepad or Wordpad, then paste the following rule into the file and save it:

scrollbar[orient="vertical"] scrollbarbutton,
scrollbar[orient="vertical"] slider {
 width: 25px !important;
 height: 25px !important;
 -moz-appearance: none !important;
}

You might need to restart Firefox to see the effect. If that isn't wide enough, you can increase the width/height from 25 pixels to a larger measurement. What will work best for you really depends on your screen resolution.

That is usually done system width via the Personalization settings.

• Control Panel > Personalization > Window Color and Appearance

You can also try a large theme to see if that helps.

• https://addons.mozilla.org/en-US/firefox/themes/large

Be sure to have the required @namespace line at the top of the userChrome.css file

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

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

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

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

"That is usually done system width via the Personalization settings.

   Control Panel > Personalization > Window Color and Appearance "

I presume you meant "system-wide," referring to the WINDOWS Control Panel, as no such location exists in Firefox. If you were talking about Windows XP or anything else before Windows 7, you would be correct about making things larger. BUT, Windows 7 does not have the options of old, where one could set Icon size, Scroll Bar size, etc. etc. (Once located in "Display.") It just isn't there any more. Any suggestions????

That should still be possible in Windows 7.
They seem to have the "Advanced appearance settings" link to access such settings harder to notice.

• http://answers.microsoft.com/en-us/windows/forum/windows_7-desktop/how-can-i-make-the-scroll-bar-wider-as-i-could-in/50a24293-aa37-4003-a66a-fd5aed510265

I switched from Aero to a basic theme. I can get the old dialog by right-clicking the desktop, choosing Personalize, then at the bottom of that pane, clicking Window Color to go directly to the dialog. With Aero themes, you get an intermediate color screen and need to click the Advanced settings link.