This Works For Firefox 56 users
(Thanks to user ae8425, for keeping my solution alive)
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Download No Squint Plus version 50.0 from here

https://addons.mozilla.org/en-US/firefox/addon/nosquint-plus/versions/

You have to choose 'Download anyway'

This will download an xpi file (nosquint_plus-50.0-fx.xpi).

Save it anywhere you want on your PC (easiest is just the desktop)

Now drag and drop this xpi file onto your Firefox shortcut on your PC desktop (even if you have Firefox running).

You will now be prompted to install the Add On

And there you go. A working No Squint Plus for Firefox 56

PS; Set updates to 'Off'
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For some reason i cant find my original review, as i posted this before, so i post this again.
So here you go, this works flawless if your willing to stay on FF 56

I find that the settings don't stick; even reloading a page loses the settings. I think I am correct in saying there is no way to create global settings.

Невероятный сайт и предложения!

Shame, with my old eyes and a 27 inch monitor, I need this. I thought when it went to NS Plus it was something Firefox acquired or was backing more than other add-ons. I will not upgrade to 57 unless this is working, period.

New installation of Firefox 57 - NoSquint Plus still won't work. It's a pity. So long and thanks for all the zoom.

All versions after 56.1 don't work. I tried them on a new installation of Firefox, with a new profile. It's the only addon that's impeding me from upgrading to Firefox 57+. Any plans to fix it? With version 56.1 I rated it 5 stars, but sadly now I rate it 0. Anybody knows of another similar addon that works in ff57+ ?

THANK YOU!!!

I've always liked nosquint, but in the last few versions before the awful 57 and now with the change to a 57+ version, it's very hit or miss. Sometimes settings per individual sites work, sometimes they don't, sometimes the interface comes up off of the menubar button and allows changing settings, but does not accept the settings -- clicking the save or the close buttons do not work and the browser must be rebooted. This behavior seems new to Firefox v.57, which brings a host of broken parts, blechh. It was not broken, now it is, good work.

this addon does not work anymore. i loved the visited, unvisited text color highlight. anyone know of a similar addon that does the same? please fix this addon for the new Firefox versions!

It simply does not work anymore, even before FF 57: I have set the default to 80%, new sites always show up at 100% and sometimes even 120%, never 80%. And in any case, when FF is rebooted, everything has to be done again.
How come that such an extension, which is pushed by Mozilla itself (how come the function is not a FF built-in one?) has become that erratic in the past weeks?
What are the alternative tools?

I have been really thankful for this add-on for a long time. Yes there's a little hick up right now, and it did not work to create a completely new Profile in Firefox 57.

However I found a workaround so I can use this til it's fixed.
It seems to only work with 1 saving/settings/site. Then I have to restart FF, and make a new setting to a site.
Hope this helps anyone else.

ThanX a million for this add-on @Baris Derin! Can't live without it!

This addon does not work. Don't waste your time on it. Even if the author of the addon makes it compatible for the current version of Mozilla Firefox browser, the developers of Mozilla will "upgrade" their browser to another version higher and the addon will stop working again. Guaranteed. It's a constant race between No Squint developer and Mozilla developers to see who can make the life of the browser's users more miserable. Both strive to win the title "The Worst Product of the Year".

Doesn't work. Tried creating a new profile, also doesn't work at all.
So don't blame users who didn't follow the instructions.
Also don't blame Baris too hard. We do not pay him for his work.

Version 62.1 is not working well. Not working at all. But: version 50.0 is working like a charm, try it if you have problems with latest versions. See it in oldest versions. To the author: I have no time to make new FF profils and copy files for ours, etc., if 50.0 is working well, why can't you able to do the latest version properly???????? Shame.

es la mejor app para firefox. la uso desde hace años y siempre ha funcionado perfecto.

If you've been using older versions of NoSquint Plus and can't get 62.1 to work it's because you haven't followed the developers instructions. Note where the developer says your Firefox profile may be corrupt and to try creating a new profile and installing the add-on. When I did this magically NSP started working like a charm.

You can create a new profile from the about:profiles page which will also show you where your old profiles are. The following support article will tell you how to copy your bookmarks, site preferences, logins/passwords etc. from your old profile.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile#w_create-a-new-firefox-profile

You can then copy over your extensions (I recommend you do it one-by-one) from the extensions and browser-extension-data folders from your old profile. Better yet add/re-install one-by-one as this is a good time to review which add-ons you actually use. You have to reapply your add-on settings and tweaks to about:config but this will reveal any conflicts that cause NSP 60+ not to work properly. All the browser.link.open_newwindow settings should be set to default in order for the global settings tab to open.

You could try just backing up and deleting or renaming the prefs.js file in your old profile. This is probably where the corruption resides. My prefs.js went from 256K to 22K between my old and new profiles. Copying my old prefs.js to the new profile caused NSP to break.

Other potential conflicts - themes or skins, the native Firefox zoom control (remove it from your toolbar if you added it), other extensions (that's why you should add them one-by-one). TabsMixPlus conflicts on pre-Firefox 57 but since there is no Webextension version yet that is not a problem for now,

The only major issues I find with v62.1 is if you click "save" settings for a site the options panel will lock up for that site only. Close all tabs for that site or restart the browser and the panel will unlock. the lack of zoom controls on the toolbar (probably due to the Toolbar API not available yet) and the zoom level indicator in the address box shows only full (text and images) zoom.

Otherwise this is an essential add-on tthat works as advertised - if you follow the instructions.

I'm not a computer guy so I don't really get what it's all about, which is hardly surprising, if there's one thing missing in I.T. it's information. Knowledge is always assumed, where you're meant to acquire it from, however, is all part of the mystery!?
Like this add-on, No Squint Plus, for example, it's gone from doing what it was supposed to do, which such is the overall standard within this field, is greeted as a major accomplishment in itself - it does the job you got it to do, "Bravo!!!" "5 stars!!!" Rave reviews on the strength of the fact that it works.
Anyway, NSP certainly doesn't work any more, it's as though we've gone back in time, to a less advanced age, what is it with updates?? The only thing that progresses in a forward direction is the date, update to a downgrade, I dread updates, it's always the same, chaos, nothing works, everything's suddenly broken, time I get things ironed out, you guessed it, it's time for another bloody update!!!!
I'm aware that people like me, who just whinge, achieve absolutely nothing + offer no solutions, but if people can knock out stuff like NSP, or rubbish, I don't think it unreasonable to label it, then I've got to be allowed to moan about it. If these people did their bit as well as I do mine, we'd all be far better off. If you need a benchmark, then look no further, the bar's set pretty damn high so aspire to it! There are some folk out there that need to raise their game, I've noticed that p.c. people don't take criticism at all well. If your work is good enough behaving like some kind of diva will fly, if it's not it won't. If the criticism is unjustified, which if I've got anything to do with it that won't be the case, which is why when I get denials, or insults, or whatever it might be? Some of the problem is right there in the attitude, usually, there is no response at all, which again is a really bad sign, if you can't get it together to send a pre-packaged "Thank you so much.....I deleted + trashed it.....Yours truly," then something's wrong, + that will be you yourself.
There's a lot i don't like about p.c. land, stuff not working, though, that's not opinion, that's fundamental, if that's wrong you deserve all you get............