Create a bookmark with the name FA user, with a keyword fa and pointing to http://www.furaffinity.net/user/%s
Then to access a particular user page, just type fa <username> into the address bar. For instance, fa captpackrat.
The important part of this is the keyword. You can change the name of the bookmark to anything you want, but you have to include the keyword.
This trick doesn't work with IE, Opera or Safari, mostly because these browsers utterly suck at handling bookmarks.