You will need to install a user script manager extension to install this script.
You may also like Fanfiction.net Unwanted Result Filter.
By default, Fanfiction.net always sets its own result filters to less-than-ideal values like "Languages: All" and "Rating: K->T".
This script adds a "Save Current Filters as Default" option to the User Script Commands submenu (or, for Greasemonkey 4, to the page's context menu) which lets you trigger an automatic redirect whenever a page with no filters specified (thus, falling back to defaults) is loaded.
(The two page loads necessary for the redirect are less than ideal, but it's still faster than changing things by hand every time and Fanfiction.net's URL scheme makes it far too easy to write an over-broad matcher for hyperlink rewriting. Suggestions are welcome.)
Goes nicely with Fanfiction.net Unwanted Result Filter to further fine-tune what results are displayed.