For firefox, I found you can use a bookmarklet to create a page of a list of links, then use download them all.
This helped if images were in a sequence, I could do say 1-1000 and download them all would download relevant images.
Doesn't always work, but it can help.
The one I specifically used was "make numbered list"https://www.squarefree.com/bookmarklets/misc.html
Set this up as a bookmark, and when on a page with images, click it - rest is pretty self explanatory.