I have a project page on my website where you can read all about it: http://www.seanster.com/screengrab/
There's more information, source code, diff's, and links to other screengrab resources.
Here's a direct link to the version 1.0 xpi file. http://www.seanster.com/screengrab/screengrab-1.0.xpi
You should remove your old screengrab before installing this one.
A sample filename with all current datestamp options enabled:
20121013-2039_notifications for preference changes • mozillaZine Forums_1350171567081_5079FBAF.jpg
Here's a shot of the new options dialog to entice you:
http://www.seanster.com/screengrab/screengrab-options.jpg

It wasn't until after I did all my work that I discovered some Russian dude recently resurrected it on AMO. I grabbed that code and merged his fixes into mine, so it's as up to date as you can get.
I'm running FF 15.0.1 and I have tested it on FF 16. I'll consider other improvements if they are worthy but I don't have a lot of time and can be easily distracted. I have other things to do.
Here's a quick summary of changes:
Added & exposed more options.
Added elite new date stamp formats of my own personal preference.
Fixed & exposed the jpg quality option.
Fixed & exposed the "status bar" icon enable/disable.
Fixed and reinstated the "toolbar" icon.
Fixed stripping invalid characters from the filename (for windows at least).
Logging options visible and working.
Added ".jpg" as a file extension option.
Option to default to download folder
Screenshot counter
Other fixes I stole but don't fully grasp.
Other fixes I forgot about.
I would appreciate feedback. I suggest you post here but you can email me if you like. I added a fair amount of English help text (EN-US) so the localizations are all out of date. Feel free to contribute new localization files.