I managed to get Nosquint Plus to work with the latest version of Firefox and to keep the settings between sessions. The trick to is to change only one setting at a time:
1. Open Firefox
2. Open Noquint settings
3. Change one setting and save
4. Close Firefox
5. Repeat 1-4 above

1) Uninstall NS+ & delete appropriate YouTube etc. cookies as well.

2) Go to: Switch to Firefox Extended Support Release (ESR) v82.0.1 (current version) for personal use...

https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr

...to "roll-back" all the extra bells & whistles that bog down the Firefox browser which interferes with the add-ons. This will take you to the download page:

https://www.mozilla.org/en-US/firefox/enterprise/#download

3) Then put:

https://addons.mozilla.org

into your cookie whitelist. (You may want to clear this cookie from time to time if you customize quite a large number of sites over time and some are out-dated for you.)

4) Then download NS+.

I just tried it an it is holding for my Win10. It is a very good add-on, the instability problem is with FF & legacy protocols.

Please use NS+ gently as it is an older build within new tech so give it time in-between adjustments to reset, especially when zooming +/-. Otherwise, bottle-necking will result in the add-on/browser crashing until you restart FF. If it didn't load on a page, wait for your system to settle down and then refresh or reload the page in a new tab.

Set all your global settings to your optimum choices and AVOID checking the individual site "exclude site for global settings" box as best you can. Keep individual site tweaks to an acceptable minimum to meet your needs. Too many tabs open at the same time will also crash the add-on, requiring a restart of FF.

5) Enjoy your NoSquint Plus

Best there ever was. WAS.

this is the only app that can work on my firefox that zoom all the pages 120%, only one! thanks.

Thanks for taking care of old NoSquint

Solve my problem.

Can only support you by rate-up.

很好用，终于不用开一次浏览器点一次缩放了，适合近视人士和眼花人士。

just installed it, and already loving it, both functionality and tweakability in prefs. Default global page zoom tested working on several sites in FF 59.0.2 macOS High Sierra. Thank you Baris!

working as of 14 Feb 2018

This is the best zoomer for sites and global settings! It is the only one that can zoom in and enlarge pics without having to scroll left/right to see the enlarged page!

I loved it before Firefox Quantum and I am so glad it is here now for the new Firefox.

супер

Невероятный сайт и предложения!