This is the closest I've found as a replacement for downthemall in FF 57+. Yes I need to prepare symlinks in advance because I typically save to external drive. I do agree with quantum's secure design to keep FF from having the ability to modify the host system but it has crippled almost every webapi download extension. This one suggests the workaround and has enough customization options so that I could mimic (and perhaps even improve) on my previous method with downthemall. The extension has successfully allowed me to take down the last block to upgrading to the latest FF by letting me simply right click to download images or links as conveniently as I used to. My only suggestions for improvement would be to simplify the options page and also have saving links or saving source part of the context menu (instead of having to keep switching them in the options page).