A good alternative to the old Save Image in Folder. A pity about the restrictions. But to work around this, as mentioned by other commenters already, you can use symbolic links. Here how to do it so you won't have to look around so much:

1) Go to your Downloads Folder ( C:\Users\Username\Downloads ). Create a folder called "Symlinks" (For the sake of having everything in one place in case you actually use your Download Folder for something)

2) Open the Command Prompt with Administrative Rights (Click on the Start Button, type "cmd", right click on "Command Prompt" and run as Administrator)

3) Use the following command to create a hard link to an existing folder you want to have as an option for downloading to: mklink *Symlinkfolder* *Targetfolder*

So if your target folder was something like: D:\Pictures\Cats the command would look like this:

mklink C:\Users\Username\Downloads\Symlinks\Cats D:\Pictures\Cats

In the Symlinks folder a Linkfile named "Cats" should appear

4) In the Options of the extension, Add the line "Symlinks/Cats" (You can also delete whatever you don't need ). Then restart Firefox so it reloads and actually takes the changes (Necessary in 1.2 it seems. Can you fix this so it changes it if you press save? )

5) Now you should have your own Options listed in the menu and it should save it to the folders specified in this way. Repeat for different folders as needed.

It would be nice if the plugin could do this by itself, but I don't know if and how that'd work with Web Extensions. (Or in the mean time, put up a guide like above into the description so people know what's possible ;) )
Also, yeah, seconding the doubleclick feature :) Also, it'd be nice to have an option where if the file already exists, it either does the renaming thing as it does now, or opens up the Save Prompt so you can rename it yourself or cancel.

Well, all in all, The closer you can get to the old plugin, the better :p

Good Job so far, though!