I found out that also on Android I have the same issue. Both are fresh installations.

Forum; Adblock Plus Homepage {web link}

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.

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 10.1.16 Adobe PDF Plug-In For Firefox and Netscape 15.10.20056
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Adobe PDF Plug-In For Firefox and Netscape: Adobe Reader Version 2015.010.20060 Note: McAfee Security is offered in the download.

Thank you for your response, but I don't have such programs. I think I found the problem, but I don't know how to solve it. It seems related to Firefox Sync, in fact as soon as I disabled it, everything worked fine. I tried to change account and to reset Firefox but nothing. To solve i just installed uBlock origin.

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.