The 'big two' are Greasemonkey for Firefox and Tampermonkey for Chrome, but there are some other lesser-known possibilities too, some of which became available relatively recently. (I originally wrote these lists in Feb 2015, the first time all these userscript-running options had been collected in one place.) Some userscripts might not work with some of the other browsers, particularly the more complex ones, but could happen with any of them if they try to use a command that browser doesn't support. (Try a script and see what happens; don't be surprised or complain if it doesn't work right, or expect the script authors to be willing or able to do anything about it, but you could reply here to let people know what did or didn't work for you in which of the lesser-known options below.) Also, most of the options other than Greasemonkey and Tampermonkey have no way to control the execution order for combinations of scripts that can be sensitive to that, and some don't support running scripts on content in iframes (so they won't work inside HITs, only on the surrounding MTurk page). Using multiple different browsers/variants that can run simultaneously allows you to do different things on or off of MTurk in separate environments, with or without having multiple computers. Except where specifically stated otherwise, this is talking about browsers for desktops/laptops running Windows/Mac/Linux. Mobile possibilities are more limited; the browsers available for mobile OSes that call themselves by the same names as desktop-OS browsers don't have the same features.