This is the Firefox hotkeys. I did find Cntrl+Enter but they don't mentioned the action that your seeing?

https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly

Open Focused Bookmark in New Foreground Tab Ctrl + Enter Open Focused Bookmark in New Background Tab Ctrl + Shift + Enter Open Focused Link in New Background Tab Ctrl + Enter (see note, below) Open Focused Link in New Foreground Tab Ctrl + Shift + Enter Note: The Foreground and Background Tab shortcuts are switched if the setting When you open a link, image or media in a new tab, switch to it immediately is enabled in Settings

Miscellaneous Command Shortcut Complete .com Address Ctrl + Enter

Where does this context menu open? Are you using Ctrl+Enter in a webpage or in the user interface (address bar) when this happens?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click "Refresh Firefox" in the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I'm using Ctrl+Enter on a web page. Attached is a screenshot from tiddlywiki.com where ctrl+enter is the shortcut to save an entry (though it happens on all pages, whether there is a shortcut or not).

This persists in troubleshoot mode (c.f. second screenshot).