Hide default context menu items in Firefox 57 and later using simpleMenuWizard

To remove entries from the context menu you need to

1. Find your profile folder (profile names are different for everyone):
   Address bar > Enter about:support > Click Open Folder.
   
2. Download this project and unzip it.
   
3. Move userChrome.css and simpleMenuWizard into [...]\profile.folder\chrome\ directory.
   If chrome folder doesn't exist, create it.
   If userChrome.css already exists, do not overwrite and proceed here.
   
4. Open userChrome.css with a texteditor for a general overview and some options.
   
5. Open the simpleMenuWizard directory and edit each .css file (except beginning with opt_) to customize them to your needs.
   Activate option to remove menu item: Remove /* at the beginning of the line.
   Deactivate option to leave menu item: Add /* at the beginning of the line.
   Each file is for another context:
   
   • blank-context.css when right-clicking on a blank area or text
   • frame-context.css when right-clicking on an iframe
   • image-context.css when right-clicking on an image
   • input-context.css when right-clicking on an input-field
   • link-context.css when right-clicking on a link
   • main-hamburger.css when left-clicking on the three lines on top right
   • main-menubar.css when left-clicking on the main menubar (open with ALT key - file, edit, view, ...)
   • media-context.css when right-clicking on media like audio or html5 video
   • select-context.css when right-clicking on selected text or object
   • sidebar-context.css when right-clicking on items in bookmark- or history sidebar
   • sidebar-header.css when left-clicking the sidebar dropdown menu
   • source-context.css when right-clicking a blank area on view-source pages
   • tab-context.css when right-clicking on a tab
   • toolbar-context.css when right-clicking on toolbar or tabbar
   • urlbar-context.css when right-clicking on the addressbar
   • urlbar-pageaction.css when left-clicking the three dots in the addressbar
6. Restart Firefox to make changes work.
   

Important notes:

• All options are disabled by default, so if you don't edit the files, nothing will happen.
• Items that appear in different contexts with the same ID will disappear in all those contexts when activated only once. This is because many menus internally share the same very big context menu and are separated here for more convenience.
  For specific problems please open an issue, there may be workarounds.

Hide Pocket / Sync / Screenshots

If you don't use either of those "internal addons" at all, you can just disable them, which will also remove their context menu entries everywhere.
How to do it: Set the respective value in about:config.

• For Pocket, load about:config?filter=extensions.pocket.enabled into addressbar and switch to false.
• For Sync, load about:config?filter=identity.fxaccounts.enabled into addressbar and switch to false.
• For Screenshots, load about:config?filter=extensions.screenshots.disabled into addressbar and switch to true.

Using simpleMenuWizard together with other custom modifications

Easiest thing to do is renaming the userChrome.css that came with the zip package of simpleMenuWizard. Rename it to simpleMenuWizard.css and put it into chrome directory next to the already existing userChrome.css.
Open the old userChrome.css which is filled with foreign code and add @import url("./simpleMenuWizard.css"); to the very top. That's it!
You can now edit simpleMenuWizard.css for a general overview and some options or continue with step 5.

Uninstall simpleMenuWizard

Delete all the files and folders that came with this project.
So if you don't use other modifications, you can simply delete the whole [...]\profile.folder\chrome\ directory.