Do you have security software that acts like a sandbox or uses virtualization to restore files on a next boot? ---- See also: *http://kb.mozillazine.org/Preferences_not_saved *https://support.mozilla.org/kb/Preferences+are+not+saved ---- It is possible that there is a problem with the file(s) that store the extensions registry. Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) and compatibility.ini in the Firefox profile folder to reset the extensions registry. *https://support.mozilla.org/kb/Profiles New files will be created when required. See "Corrupt extension files": *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.org/kb/Unable+to+install+add-ons If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is a compatibility update available.

Dear "cor-el" Thank you for your attention, but i couldn't solve the problem yet. I have the same Security software and the same extensions installed, on my laptop and desktop, but there is no problem on my laptop. Have you any suggestion rather than you prefer ?