If you disable MultiProcess and restart FireFox Save Images addon will work again.
This info was gained from the below support/hack site which goes into much more detail and the risks and how to backup your profile.
Check if Multi Processing on by typing
about:support and check the Multiprocess Windows will be set to True
To enable or disable multi-process Firefox, do the following
Type about:config in the browser's address bar.
Confirm that you will be careful.
Search for browser.tabs.remote.autostart.
Double-click on the preference. (it will then set as False)
Setting the value of browser.tabs.remote.autostart to true enables the multi-process architecture in Firefox, setting it to false disables it.
Restart Firefox and you can check that it False by typing
about:support (note it is now False)
Test your Save Files and Bingo!
Excelent!!! Rated 5 out of 5 stars
Perfect app!!This review is for a previous version of the add-on (188.8.131.52).
thank you Rated 5 out of 5 stars
i install v48 and save image work again..This review is for a previous version of the add-on (184.108.40.206).
Firefox 51 Rated 5 out of 5 stars
It doesn't work, please fix (This review is for a previous version of the add-on (220.127.116.11).
bug Rated 3 out of 5 stars
There was a problem saving the image.
You may not have permission to write to the storage folder or the file name is wrong. Please check the permissions of the folder and check the file name option.
Can not save any data
When will it be compatible Rated 5 out of 5 stars
When will this add on be compatible with FireFox 51? We use this add on every day and need it badlyThis review is for a previous version of the add-on (18.104.22.168).
"Save Images" does not work correctly with versions of Firefox greater than 48 due to the latest versions of Firefox using multi process functionality. If multi process is disabled, it should work correctly but I cannot guarantee that it will.
Until the developers of Firefox have sorted out all the changes they are making, I will not be updating Save Images.
Trying to upgrade "Save Images" to work now would require losing 80% of is current functionality, which is not worth the effort.
Not working since Firefox 51.0.1 Rated 1 out of 5 stars
Same problem as kkllkl (29.01.2017)This review is for a previous version of the add-on (22.214.171.124).
Error in use Rated 1 out of 5 stars
There was a problem saving the image(s).
You may not have permissions to write to the save folder or there is an error with the file name. Please check the permissions of the folder and check the file name options.
The above message appears while using the function to save the image.
I do not know how to solve it.
Works fine with Firefox 50.0.2 Rated 5 out of 5 stars
Don't know why the page says that this extension isn't commpatible with FF48+. I just installed it and used it with no problems whatsoever. My copy of Firefox is 32 bit, 50.0.2.This review is for a previous version of the add-on (126.96.36.199).
Once, the greatest of all Rated 4 out of 5 stars
For now, as I've browsed some of the answers here by Lumox, I just came by to say a huge THANK YOU. This extension saves time, does its job gracefully, was once uncomparable for saving images. My "relationship" with it was of a child: initially, I just wanted for saving images from the current tab. Then I slowly got into the more advanced settings.
The Firefox extensions scene became obscure for quite some time already, and this frustrates most of the best extensions devs. One really needs to LOVE Firefox to keep up with so many confusing changes.
Today, there are some better extensions for the same task and are compatible with the newest Firefox changes. All in all, I totally understand Lumox: patience has its limit.
I will upgrade Save Images once the developers of Firefox have a more substantial code base. At the moment, a lot of Save Images functionality would be lost because it is not yet supported.
Which other add-ons are you using in replace of Save Images? I am curious to see what they are capable of and what extra features they have.
I loaded this add-on and it worked beautifully on small sites, BUT Rated 3 out of 5 stars
when you get to the larger ones, the windows that pops up you have to check and uncheck boxes is really difficult to use. It does not allow the "group" using the shift key for selection and most of the time, you have to manually select each one of hundreds that do not apply what you want to save.This review is for a previous version of the add-on (188.8.131.52).
Отличное дополнение Rated 5 out of 5 stars
Всё работает, всё очень удобно, хорошая функциональность. Спасибо.This review is for a previous version of the add-on (184.108.40.206).
Love it (but it no longer works) Rated 5 out of 5 stars
5 stars because it has been a great add-on for a long time. Recently however, it has stopped working.
When attempting to "open ALL image links in 1 tab" it shows the links to open but then, instead of opening a new tab and loading the images, it just switches to the right-most tab of the window and does nothing.
Running Win7 - I updated to FF 49.0.2 but no change.
Interestingly, the "open ALL image links in TABS" works!
have to be reinstall every day Rated 1 out of 5 stars
Since about 10 days, it does not work anymore. I must each time install again the app, and I lose the time it make me save. If it continue I will stop using it, Pity as it was the best app to save many pic by a simple clik. As written by konart hereunder, if I do not reinstall each time i want to use it, All saved images are 0 bytes.This review is for a previous version of the add-on (220.127.116.11).
used to be better Rated 4 out of 5 stars
This extension has been on my must have list for a few years now. It saves a lot of time when I absolutely must have all the pictures!
But now, when I tell it to DL all the pictures, it puts them in folders... wth?
If you have the tree structure selected (default, BTW) it will put your pictures in individual folders 3-7 layers deep!
So now instead of saving so much time with this extension, I'm now spending time taking the pictures out of the individual folders (Who the hell does that?) and putting them where I want them instead.
I probably would be saving a lot of time just DL'ing them individually and putting them where I want them in the first place instead of re-organizing after the fact.
Edit: Mr Lumox here solved my issue for me, and I have revised my rating and wanted to note this here. I don't know how that box was checked, but I can quite honestly say that I didn't manipulate it. If there are any further issues, i will contact you at the email provided. Thank you very much for the assistance.
Nothing has changed in the way the folders are created. The only way multiple folders are created is if you have the setting "Append image URL folder structure to 'Save Folder" checked in your options.
Please export your "Save Images" settings by clicking the "Export" button in the "Save Images Options" window and send me the file and I will check.
+1 with Seamonkey Rated 5 out of 5 stars
Wonderful as stated previously, and Seamonkey users love you long time.This review is for a previous version of the add-on (18.104.22.168).
powerful and effective Rated 5 out of 5 stars
Been using this add-on for years, it's SO useful. Never had any problem with it. Thanks!This review is for a previous version of the add-on (22.214.171.124).
Does not wort Rated 1 out of 5 stars
All saved images are 0 bytes.This review is for a previous version of the add-on (126.96.36.199).
Awesome image saver, but not e10s compatible. Rated 4 out of 5 stars
This is an incredibly handy addon for saving the images in a webpage, and has lots of options to customize where to save, how to name images, keyboard shortcuts, etc. It's saved me a ton of time where I normally would have saved images one by one.
The only problem (that's not a huge deal currently, but will become so in future versions of Firefox) is that it's not e10s compatible. Hitting the shortcut to save images in the current tab brings up an error dialog saying there was a problem saving the images (supposedly because of a lack of permissions or incorrect file names) and creates empty, 0 byte image files. I'm not sure if this is something that can be made to work with e10s, but I'm hoping so, since this is one of my favorite extensions.
time saver Rated 5 out of 5 stars
but it takes a lot of RAM on multiple tabs, and open all links in tab shortcut keys is not working, but still a time saverThis review is for a previous version of the add-on (188.8.131.52).
I have tested the keyboard shortcuts and it works correctly.
Firefox needs to be restarted for the keyboard shortcuts to work. If it still does not work, the keyboard shortcut you have chosen might be used by Firefox itself or another extension you have installed.