contextOptions

object. Options for how the context menus will be overridden.

showDefaults Optional

boolean. Whether to also include default menu items in the menu.

context Optional

string. ContextType to override, to allow menu items from other extensions in the menu. Currently only 'bookmark' and 'tab' are supported. showDefaults cannot be used with this option.

bookmarkId Optional

string. Required when context is 'bookmark'. Requires 'bookmark' permission.

tabId Optional

integer. Required when context is 'tab'. Requires 'tabs' permission.

Open the tab context menu on your custom UI, in this case:

document.addEventListener(
  "contextmenu",
  (event) => {
    const foo = event.target.closest(".foo");
    if (foo) {
      // When the context menu is opened on an element with the foo class
      // set the context to "opening a tab context menu".
      browser.menus.overrideContext({
        context: "tab",
        tabId: parseInt(foo.dataset.tabId),
      });
    }
  },
  { capture: true }
);

See this blog post for more details.