Thank you Canapy!
I used this wonderful add-on on a FF version older than 57(after which, basically our experience in FF became more or less painful).
It gets the job done! How? Let me tell you. Well, with a bit of help from another wonderful good old FF extension called Flashgot(it seems that Mozilla removed this extension from their add-on website, like it is the case with many extensions that could not be ported to those damn webextensions).
So, here are the steps:
0.make sure you have set in the Image Downloader preferences the Deep Scan(extremely IMPORTANT feature that can pick original size images that are behind thumbnails{kind of you see a thumbnail image that is actually a webpage that contains a link to the original image}) and chosed a minimum width(height) of images from hyperlinks{I have chosen 500}
1.Click Image Downloader button, then click Current Tab
2.The extension is very helpful showing you the resolution of the picked images(and a direct link to the original image), so you can select with left click every image you want. After that you click on "Save to Dash board" (it is a button on the right top corner. (Save to folder did not worked for me at all, but even if it did, considering the limitations that Mozilla imposed(the fact that add-ons can only download under Downloads folder, and cannot let the user select the desired location), would have been for nothing.
3.Click Image Downloader button, then click "Dashboard". In the page that opens, right click then FlashGot All, then you can select the desired location download the images.

P.S. Just a thought for the developer: if you could somehow send the selected links from the ImgDwnldr_Menu, to another downloader(another extension or program outside of FF) which for sure let us select the download location !?
Cheers!