This issue is most likely related to the Save File to addon that you have installed on your version of Firefox.

To determine if the issue is caused by an addon that you have installed, please try running Firefox in Safe Mode. This is a diagnostic mode that will disable your addons temporarily. If the issue is not present in Safe Mode, it is probably caused by a addon.

For more information on running Firefox in Safe Mode, please refer to this Mozilla support article:

• Diagnose Firefox issues using Troubleshoot Mode

Thanks for the tip about Safe mode. Save Image As.. certainly does work in safe mode.

As to "Save File To". No cigar. I have now removed this add on BUT I ADDED it THINKING it would solve my Save Image As, issue which it didn't.

I should have mentioned this and I do thank you for your reply.

Do you think that my inadvertent activation of Filer Keys (Holding SHIFT down for more than 8 seconds) might be a red herring? Right Click <anything else> still works so there must be a missing file (in my ignorance).

Looking again at your response, I have not added any new add-ons for over a year. I am very careful about adding rubbish to my PC.

I COULD disable ALL my add ons and try again. I am assuming that disabling an add on, if a particular add-on was the cause, would rectify the issue or do you think I should UNinstall them all?

If I do that, maybe I should reset firefox though having never done this before, it does terrify me mildly as I didn't realise how much I rely on my saved passwords (I have them written down), and book marks etc etc.

I suppose I have 2 choices... Spend the time seeing which add-on (assuming an add on is indeed at fault), or Reset Firefox and be done.

if you can think of anything else I should try, I would appreciate your thoughts even if it is to say "Reset it".

Saving my profile is pointless as I assume that somewhere in there is the corrupt file. I really hate computers BUT... this is the first Firefox related issue I have had in many many years.

I COULD simply save my e-mail folders and load up a backup image though I would like to know the problem rather than just replace the entire planet. I won't learn anything that way...

IF you have any more guidance, I am happy to try stuff out.

SOLVED IT.

I disabled ALL add on's and Save Image as.. works.

I then enabled all the add on's one by one until I found the one that kills the feature.

MOZILLA ARCHIVE FORMAT 3.1.3 is the culprit.

I have no clue as to how to properly disseminate this to the author or the user group.

I shall TRY to locate a replacement add on that allows Save Image As.

I have enabled and disabled this add on many times to confirm my findings. I shall check ,my laptop for this add on as THAT still downloads images properly. There MAY another contention I am unaware of.

I would hear any comments before I CLOSE this question. Weapons Grade Respect.

This extension just updated yesterday:

Mozilla Archive Format, with MHT and Faithful Save

You can try using the button on the Add-ons page above the Extensions list -- has a gear/wheel/cog icon -- to Check for Updates to see whether that picks it up.

According to one of the reviews, the beta test version of 4.0 worked properly in Firefox 46, so hopefully the final release will as well:

Temporary Solution for Compatibility with FF46
by AHO on April 27, 2016

Use Beta version 4.0b6.
Donwload link at the bottom of this page.

https://addons.mozilla.org/firefox/addon/mozilla-archive-format/reviews/

I updated my errant i7 machine and it all 'came good'.

My laptop was also using 3.1.3 but that was OK with Save Image As.. so I am not sure what was going on.

Thanks all round for your assistance. Weapons Grade Respects