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.

Well, I tried every possible solution described in link you provided. And nothing helped to this date. I also reinstalled my Windows as a last try but i didnt work either

Do this as a test.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Open and close it several times like this.

Is the problem still there?

Yesterday I started firefox in safe mode about 20 times and everything worked perfectly. But when i started right now it happened again. I dont know what safe mode should do, but it didnt help.

Well if Firefox works in its safemode but not ordinarilly it gives us clues as to what may be happening.

It is quite possible it is a Firefox extension casing the issues.ft

Quite possibly it is one of your Firefox extensions interfering. Please trial with all extrnsions disabled, and reenable them one at a time. Does that hrlp identify the problem?

So for now i disasbled all extensions. Will provide more informations as soon as the issue will appear again.

I think I found the issue. After turning off "OpenH264, provided by Cisco system" I havent recorded appearing the restart for about 2 days now. So i think that was the problem.

Glad you seem to have solved the issue & thanks for posting back.

It seems odd that that add-on interferes with use of Sync. I believe that add-on is usually only used if you are streaming specific types of Video.

• Why is there an OpenH264 plugin in Firefox?

It is also worth noting that Firefox 50 has an issue at present with Sync if Private Browsing mode is used, or the Never Remember History option. It is hoped that issue may be fixed soon (Hoping for Fx50.1 early December)

I will nark your post as the solution.

Thank you for your help and patience in resolving my issue. Have a good day.

Do you have updates settings for automatic updates?

I disabled automatic updates on all plugins, and then disabled for this specific one.