Did you check to see it works for the latest version of firefox? See what the addon developer says what browser version it works with.

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.

Thanks for the suggestions, but none have worked. I am using the very latest FireFox version (66.0.1). The theme was recently updated. Although I do use Advanced System Care, I set it so it does NOT automatically load at boot, so it is inactive.

Interestingly, I noticed that when FireFox loads, initially the desired add-on theme is active, but just a few seconds into the loading process, it changes to the default.

'Tis a puzzlement!

BKSmith714 said

. . . using the very latest FireFox version (66.0.1).
The theme was recently updated

Check with the theme's support.

when FireFox loads, initially the desired add-on theme is active

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Now in the Reply Box on the forum page, do a right-click in the box and select Paste.

This will show us your system details. No Personal Information Is Collected.

Name: Firefox Version: 66.0.1 Build ID: 20190322013140 Update Channel: release User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0 OS: Windows_NT 10.0 Multiprocess Windows: 1/1 Enabled by default Web Content Processes: 3/8 Enterprise Policies: Inactive Google Location Service Key: Found Google Safebrowsing Key: Found Mozilla Location Service Key: Found Safe Mode: false

Crash Reports All Crash Reports Firefox Features Name: Firefox Screenshots Version: 35.0.0 ID: screenshots@mozilla.org

Name: Form Autofill Version: 1.0 ID: formautofill@mozilla.org

Name: Web Compat Version: 3.0.2 ID: webcompat@mozilla.org

Name: WebCompat Reporter Version: 1.1.0 ID: webcompat-reporter@mozilla.org

Extensions

Name: Adblock Plus - free ad blocker Version: 3.5 Enabled: true ID: {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}

Name: Disable HTML5 Autoplay Version: 2018.10.3 Enabled: true ID: disable-html5-autoplay@afnankhan

Name: Form History Control (II) Version: 2.3.0.0 Enabled: true ID: formhistory@yahoo.com

Name: LastPass: Free Password Manager Version: 4.26.0.4 Enabled: true ID: support@lastpass.com

Name: Screengrab! Version: 2.18 Enabled: true ID: {02450914-cdd9-410f-b1da-db004e18c671}

Name: Classic Theme Restorer Version: 1.7.3.1 Enabled: false ID: ClassicThemeRestorer@ArisT2Noia4dev

Security Software

Type: Norton Security Suite Type: Type: Norton Security Suite Graphics

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You should remove Classic Theme Restorer. It is not compatible with Quantum Firefox.

Have you contacted that theme's support?

Thanks for the suggestion, however that didn't fix the problem. I removed Classic Theme Restorer and restarted FireFox, but the theme reverts back to default after about 5-6 seconds of restarting.

Same problem here. The theme changes, but clicking away and back to Firefox changes the theme back to the light theme. I updated tonight (to 70.0b3) and that's when the behavior started.

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.

I just fixed it by disabling another addon (which I didn't check if you have but if someone runs into this, I want them to find what worked for me) which gives you a dark interface while keeping the Windows dynamic color for your tab-bar and some other interface elements. it was called "Native Dark" - it appears to have fixed my issue.

Yeah, fixed on mine, as well. I also had some other extension installed that was enforcing a dark theme.