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how to edit the right-click context menus in FF

Install Menu Editor the same way you would any other Firefox

Add-on, and then restart Firefox. 

   Scroll down to the Menu 

Editor entry, select it, and click the Preferences button.

   The main Options window
for Menu Editor will open.

https://appuals.com/fix-right-click-not-working-on-firefox/

That does not solve my problem...

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

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 for changes to take effect.

• 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

I found the issue, it seems to be a bug:

I am running macOS 10.15.2 / Firefox 71.0; To reproduce:

1. Make sure you have multiple virtual desktops opened (mission control) 2. Go to "Desktop 1" and open Firefox (i used a new/clean profile) 3. Right click the opened Firefox icon in the Dock, and choose "Options - Assign to -> This Desktop" 4. Now maximize the Firefox window by clicking on the green dot to enter full screen (this will move Firefox to Desktop 2 or 3 or above) 5. Now, if you right click in any webpage, the context menu that should pop up (the one that contains "save page as" etc), will *not* appear, as if the right mouse button is not working. Also, if you left click any *folder* in the bookmark toolbar, the popup window with the items in that folder will not appear.

To show what's going on: in step #5 when you right click (and the popup window does not show up), open Mission Control (in my case it's swiping up with 3 fingers on the trackpad) to show the overview of all the desktops, and you will see that while Firefox is opened Fullscreen in Desktop 2+, the right-click context menu is opened in Desktop 1 which we assigned Firefox to in step #3.

The workaround is to NOT assign Firefox to a specific Desktop.