Do you not need to use a 3rd party program for that to work like you want it to ?

Was hoping it could be done by firefox but maybe that's the solution. Can u recommend a 3rd party program?

Have you followed the directions in this to mirror : https://support.apple.com/en-ca/HT202351 I found that through : https://www.google.ca/search?q=dual+monitor+for+mac&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&dcr=0&ei=70u0WvfwG6mh-gTFvLu4DQ See if anything else useful.

Thanks but that's not my problem. My problem is when I go to full screen on 2nd monitor, the firefox menu bar disappears on main monitor. I can get the apple menu bar on main monitor but not the firefox menu bar when 2nd monitor is full screen.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make the change effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Thanks but no luck. I just got flashing/stuttering video, no change to the firefox menu bar problem. Reminds me of a problem about 10 yrs ago & the fix was "make sure you have hardware acceleration ON. Think my prob is something else. Maybe a hardware issue? Does this happen to you when going full screen on 2nd monitor??

The menu bar in full screen is suppose to disappear and some where read can mouse to it and should reappear. Then disappear.

As to what/how/why/where no idea.