As mentioned in bug 34572 comment 29, some users use custom userChrome CSS to hide extension items from the page context menu. Bug 1700732 attempted to read the user styles and respect them, but the approach proved to be harder than anticipated.
Instead of rolling heuristics, an alternative approach could be to add a pref that allows users to enable/disable extension items in the context menu.
+1 to this! Would be awesome if it was an array of extension IDs rather than just a boolean toggle to turn all extensions on/off, as there are some I want in the context menu and others I don't. Fine by me if it's just an about:config pref and doesn't have a place to change the settings in the UI.
While it's better than nothing I wanted to at least leave the note that it will only cover a small part of the user's requirements. In my experience as operator of the biggest Firefox support platform in the Germans speaking regions there is a much bigger desire to hide some built-in features than add-on items from the menus. Add-on items are kind of expected for most users since users install these add-ons for a reason so there is a big chance that if such an add-on adds a context menu item it adds some value for the user. By the way, users not only want to hide items, they also change the order of menu items, they add icons and/or change colors. So I really hope that the resolution of bug 1700732 will be reconsidered, even if at a later point.