The Tab warn items in "Options/Preferences > General" are hidden when the related prefs have the default value and warnings are enabled. They only show when you set the related prefs on the about:config page to true.

Note that this requires to close and restart Firefox to make changes apply fully.

• "Warn me when closing multiple tabs" (browser.tabs.warnOnClose: true)
• "Warn me when opening multiple tabs might slow down Firefox"

Hello and thanks for quick reply...

Well, after a restart of computer tonight, warnOnClose checkbox is back, don't ask how :/

Looks strange as i always used this option, so my config was long time set and the warnOnClose is gone recently, probably after the Stylish extension installation that i suspect now to be responsible for small changes to my config...

Could it be possible ?

Thanks