With this extension, holding either [CTRL] or [command] (meta key) when you hit [ENTER] will cause the new tab to be opened but not switched to. I find this crucial for when I want to keep reading a web page but want to make tabs in the background for later reading (e.g. looking up words, starting searches form the search bar, simulating a "CTRL-click" on a link without needing the mouse, etc.) This extension will let you open all those tabs in the background and not have to do all the annoying switching of tabs to get back to where you were. It works from either the URL bar or the search bar.
It's very handy to use this in conjunction with search engines that you have a "keyword" set up for. I have "o" for "onelook.com" (a dictionary lookup site). If I see a word I want to look up, i type: CTRL-L "o thewordiwanttolookup" CTRL-ENTER and a tab opens in the background for later reading, no mouse needed, no interruption of what I'm currently looking at.
In the past, it has seemed to be incompatible with Tab Mix Plus, but a recent user report seems to show that it is compatible -- says the user, 'it may have to do with the inverse tab focus setting in tab mix plus'. Not sure about compatibility with Tab Mix Lite.
It also works when CTRL-clicking on the search button and the button on the URL bar (middle click the URL bar button to switch to the new tab as well.)
Normally, when you enter something in the URL bar or search bar and hit [ENTER], it replaces the current tab's contents, and if you hold down [ALT] when you hit [ENTER], it will open the contents in a new tab, and it will switch to that tab. I wanted a third functionality using the CTRL key.
If you use the extension Stay Open Menu you will probably want to go into the preferences for that add-on and disable it for the "Smart Location Bar" or some key combinations may not work.
People used to the IE-style where the [CTRL] causes prefixes/suffixes to be added to the typed-in phrase will notice that functionality gone.
Note: there is a hidden browser preference called "browser.search.openintab". It defaults to false. If you set it to true, then the behavior of this extension stays the same, but the normal modification reverses with respect to [ALT], in terms of the search bar.
(The setting available at Preferences->Tabs->"When I open a new tab, switch to it immediately", a.k.a. "brower.tabs.loadInBackground", which affects the behavior of [CTRL]-clicking URLs in pages, may also play into things, but I don't think so...)
Possible future features:
* the ability to change which key combinations do what
* the ability to have the default (no key) action be changed as well