This will disable add-on signature enforcement; Enter about:config into your Firefox address bar Click the I'll be careful button Find xpinstall.signatures.required in the list Right click the preference Select Toggle until the value is false https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox

https://wiki.mozilla.org/Firefox/AddOns/Status/Updates#Exec_Summary “Currently targeting Signing with no pref to turn off in Firefox 47”

If I toggle true to false, the Status changes from default to User Set; BUT the value changes to false. Either way, it still does not allow the add-on.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

No, I have no such software installed. It seems to me that since User Set status is "false", and that altering value to true merely reverts status to default, and that User Set status reverts to default on restart, Mozilla developers have simply determined that unsigned add-ons will no longer be permitted, whether the user wants them or not. A possibly cavalier attitude, but perhaps understandable in the interests of security. I am more annoyed with McAfee who are no longer (according to their "support") supporting Safekey, despite customers having paid for it. They have replaced it with a new product "True Key" which will work in Firefox, but which I have to pay for if I want the same level of product that I had with Safekey, included with the Anti-Virus suite. For now, I have given up on Safekey and am using the free version of True Key (just 15 passwords allowed) pending ditching McAfee altogether. Their attitude towards paying customers really is cavalier, and will lose them business. At least I've had one cheap year out of them! - And remembered to switch "Auto-Renew" from "On" to "Off" in their account settings. They set it to "On" by default so people get conned into the second year at full price. Did I say "cavalier"?

Did you check out the link?

I had already followed links to this page, and decided a) that it looked too specialist and complicated for a layman, b) that I don't have the software referred to, c) that I had installed no new software - this is prior to True Key - or altered settings and d) that someone in work doesn't need another full-time job maintaining software that only no longer works as it did because its developers had meddled with it. I have come to dread updates, whether to android apps on my tablet, to Windows, or to any of the applications I use on my laptop, because about every two months something gets screwed up. However, I am free today so I will try some of the suggestions, at least. Here's hoping!

I have now followed instruction as far as reinstalling firefox from scratch with no change in outcome. I also made suggested alteration in about:config preference. At least it persisted this time, but still did not allow installation of Safekey.

Thursdaycat said

I have come to dread updates

I know the feeling. When a new version of Firefox comes out, I normally wait until I see a reaction from users here.

BTW I notice that my version of Firefox is now 43.0.4. However, I think I've probably wasted enough time on it. Thanks for your attempts.