I suppose you are familiar with toggling the extensions.screenshots.disabled preference to true in order to disable the feature entirely, which is not what you want.

To hide the context menu entry only, try adding this to your (or create a) userChrome.css file: @namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul); #context-navigation, #screenshots_mozilla_org-menuitem-_create-screenshot { display: none !important } (Source, including important remarks and a link to userChrome.css help)

Note that the entry will still be enabled in the address bar’s three-dot menu, which may be no issue. If it is, use this for the second line: #context-navigation, #screenshots_mozilla_org-menuitem-_create-screenshot, #pageAction-panel-screenshots

That bottom section of the context menu is used by all extensions that create new menu items, so it may be difficult to keep it clear in the long run. You can use a custom style rule to force the Inspect Element item and its accompanying separator to the bottom of the menu so that regardless of how many extensions add to it, you will always find it in the same place.

/* 
   Move "Inspect Element" and preceding separator 
   down the menu
*/
#context-inspect, #inspect-separator {
  -moz-box-ordinal-group: 20 !important;
}

For more information on setting up a userChrome.css file, see: https://www.userchrome.org/how-create-userchrome-css.html

I compiled a list of the ID's of all the CSS context menu selectors. This includes all of the separators.

I figured it may be useful to someone, especially since I was not able to find all of these in one place and had to follow a couple separate posts on the internet in order to figure out what every thing is called. As a note, these ID's are current as of Firefox ver. 61.0.2 (64-bit)

Removes Items from Tab Context Menu

#context_reloadTab
#context_toggleMuteTab
#context_pinTab
#context_unpinTab
#context_duplicateTab
#context_openTabInWindow
#context_sendTabToDevice_separator
#context_sendTabToDevice
#context_reloadAllTabs
#context_bookmarkAllTabs
#context_closeTabsToTheEnd
#context_closeOtherTabs
#context_undoCloseTab
#context_closeTab

Removes Separators from the Tab Context Menu. Keep in mind that for each separator below there are multiple names I was able to identify that work. You only need to copy one item in order to remove the specified separator.

1st separator
menuitem[label="Mute Tab"] + menuseparator
#context_toggleMuteTab+menuseparator

2nd separator
menuitem[label="Move to New Window"] + menuseparator
#context_openTabInWindow+menuseparator

3rd separator
#context_sendTabToDevice_separator
#context_sendTabToDevice+menuseparator

4th separator
menuitem[label="Close Other Tabs"] + menuseparator
#context_closeOtherTabs+menuseparator

5th separator
menuitem[label="Close Tab"] + menuseparator
#context_closeTab+menuseparator

Removes Items from Right Click Menu; Diagram here: https://imgur.com/b5gEfUy

#context-back
#context-forward
#context-reload
#context-stop
#context-bookmarkpage
#context-navigation
#context-sep-navigation
#context-savepage
#context-pocket
#context-sep-sendpagetodevice
#context-sendpagetodevice
#context-sep-viewbgimage
#context-viewbgimage
#context-selectall
#context-sep-selectall
#context-sep-viewsource
#context-viewsource
#context-viewinfo
#inspect-separator
#context-inspect
#contentAreaContextMenu > menuseparator:nth-child(92)
#screenshots_mozilla_org_create-screenshot

Removes Items from Right Click on Selected Links Menu; Diagram here: https://imgur.com/e9AaMx3

#context-openlinkintab
#context-openlinkinusercontext-menu
#context-openlink
#context-openlinkprivate
#context-sep-open
#context-bookmarklink
#context-savelink
#context-savelinktopocket
#context-copylink
#context-copy
#context-sep-selectall
#context-searchselect
#context-sep-sendlinktodevice
#context-sendlinktodevice
#context-viewpartialsource-selection
#inspect-separator
#context-inspect
#contentAreaContextMenu > menuseparator:nth-child(92) This is a Separator