Do you mean the item in the Help menu (don't know if on Mac it is elsewhere)?

On Mac the main menu bar is under control of the Mac OS and you can't use code in userChrome.css to hide this entry (#helpSafeMode).

Keep in mind that you only have to hold down the Options key on Mac (Windows: Shift key) when starting Firefox to start in Safe Mode.

Note that you also have buttons on the about:profiles and about:support pages, but you can hide these with code in userContent.css

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

@-moz-document
 url-prefix(about:support),
 url-prefix(about:profiles) {
   #action-box {display:none!important;}
}

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor (Windows: Notepad; Mac: Textedit) to create a (new) userChrome.css and userContent.css file in the chrome folder (file name is case sensitive)
• paste the code in the userChrome.css and userContent.css file in the editor window
• make sure that the userChrome.css file starts with the default @namespace line
• make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css and userContent.css (case sensitive)

Yes, I'm on Mac os High Sierra version 10.13.01 ! Does that change any of the above instructions?

It doesn't change anything, but like I wrote, you can't remove/hide this item in the Help menu (or Firefox menu if it shows there on Mac). Modifying the main menu bar like can be done on Linux and Windows via userChrome.css is not possible on Mac.

Ah yee, the place I need to hide it is in the Help menu @ Restart with add ons disabled. Thank you regardless though.