The Shelve extension provides a way to quickly save the current document to a predefined folder defined by means of a filename template.

Filename templates

The template string can contain placeholders for the document’s host, filename, date, clip, and more.

Hotkeys

Shelve can be invoked by a user-configurable hotkey (default: F9). Users can assign hotkeys to individual “shelves”.

Logs

The downloaded document can be registered in a log.

Footers

A custom footer, which could contain the source URL and an optional note, can be appended to the local copy of the document.

Auto-save mode

Users can enable auto-save mode when invoking the main shelve dialog. Alternatively, they can set a shelve to which all documents should be saved to after startup.
In auto-save mode, every subsequently loaded document will be saved to the same shelf (%c clip will be disabled though). The status is reflected by the toolbar button’s state. To switch auto-save mode off, invoke the main shelve dialog again, e.g., by pressing the main hotkey (F9).

There already exist several extensions that try to solve not all too different problems. I personally used slogger for quite a while but it seems to be currently unmaintained and it is too heavy-weight for my needs.

A non-comprehensive list of similar extensions (one way or the other). If you’re missing a feature in shelve, you might want to take a look at the following extensions:

slogger

Currently unmaintained. It also provides some integration with social bookmarking services.

scrapbook

Save pages (or clips) in a local archive. The use of an archive makes it difficult to sync saved webpages with tools like rsync, unison etc.

save file to

Re-build the directory structure in the context menu. There is no way to create a new directory.

auto save document

Saves pages as you visit them (not tested).

Automatic Save Folder

This add-on will open the explorer window at the right location working with simple filters set on the downloaded file.

Other useful plugins:

nuke anything enhanced

Prepare the webpage before invoking shelve.

highlighter

Highlight text

Mozilla Archive Format

Shelve provides experimental integration with the MAF add-on (introduced in version 1.22), which allows users to save webpages in MHTML or MAF format