This add-on is a rewrite of the now unmaintained TabSubmit (which is, ironically, a rewrite of the still older SubmitToTab).
It's very stripped down. Once you've installed the add-on, it should inject a page-mod that causes any form to target _blank when you click on any input and hold down a meta key (like Cmd, Ctrl, etc). That should, in theory, open it in a new tab or window, depending on what your browser.link.open_newwindow pref is set at.
Made during a bout of insomnia for RyanVM, because that guy saves our butt daily.
* Added intentional middle-click support (middle-click used to work, but only because we were taking advantage of some unspecced behaviour that got "fixed" in Firefox 54)
