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.

Hi,

Thanks but I don't think it matches my case. Prefs are ok, as menu bar doesn't appear after restarting, so my settings are ok.

Seems you are using the Classic Theme Restorer extension? There may be a glitch in how it -- or another add-on -- is moving the tab bar from its normal position above the main navigation toolbar, or maybe there is an extra toolbar??

I do not use CTR, so don't have much insight into how it works, but I believe you can "live" test what the various selections and checkboxes do to discover whether that helps. You just need to start by clicking its Options button on the Add-ons page, Extensions list. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then find Classic Theme Restorer on the right and click its Options button.

Note: CTR updates frequently, so you might also check for updates by clicking its More link, scrolling to the bottom, and using the Check for Updates link.

Thanks jscher2000, after checking/unchecking some checkboxes with no result, I loaded default classic settings in CTR, and it worked !