usi (User|Unified Script Injector) sɣur USI

Now you can use your Userscripts on your Firefox for Android, too.

Firefox 57+
Please install USI in version 0.5.1.x for usage in Firefox 57 and above.

If you find any Bugs, please create an issue at https://bitbucket.org/usi-dev/usi/issues/

• experimental Greasemonkey functions (GM_getValue,GM_setValue,GM_deleteValue,GM_listValues,GM_setClipboard,GM_openInTab,GM_log,GM_addStyle)
• you can add a Userscript through a simple Textbox on your device, or you load it by an URL
• if you open a Userscript in it's own tab with an ".user.js" ending, USI will ask if you want to import this Userscript
• you can use jQuery in your Userscripts (@include-jquery)

Note:
This Addon is still an alpha version, many things will not work properly.

Userscripts written for Greasemonkey won't run sometimes, because of the handling for @run-at defined in the meta block.
Try to change it from "document-start" to "document-ready" if some Userscripts does not run. For more Information about this topic, take a look on this page where contentScriptWhen is explained (https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/page-mod)