Just tried it – pretty simple and convenient.

“Save links, images and web pages to user-defined folders and their sub-folders straight from context menu.”

Source: Save File to :: Add-ons for Firefox
  https://addons.mozilla.org/en-US/firefox/addon/4902

Review @ Lifehacker

“Firefox: If you hate having to search through your folders every time you change the type of file or directory you’re saving to—and we certainly do!—Save File To helps you make custom context menus for file saving bliss…”

Source: Save File To Creates Custom Save-As Context Menus in Firefox – context menu – Lifehacker
  : http://lifehacker.com/5480003/save-file-to-creates-custom-save+as-context-menus-in-firefox