Must-have. (core material) Rated 5 out of 5 stars

UPDATE:
To simplify installation of dragdropupload, I've patched its install.rdf as outlined below and re-packaged it as xpi:

http://www.unleashedmind.com/drop/dragdropupload-1.9.38-fx-compat.xpi

Note: Installing extensions from arbitrary sources on the net is not necessarily secure. I've only fixed the extension's Install Manifest file and corrected the filename of its icon, so it appears in the Add-on manager.
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To make the dragdropupload extension work in latest Firefox versions:

1) Locate the extension's directory in your Firefox profile; e.g.:./extensions/{CB56AAF9-68C8-41bd-8E5C-7B53232CF7B9}/

2) Open the file install.rdf within in an editor.

3) Locate the section for Firefox, which is id "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}"

4) Change the value of "maxVersion" to "18.*" (or higher).

5) Close and restart Firefox.

If it does not install, additionally change the extension em:id to: dragdropupload@teslacore