Unified Script Injector (usi) is a new Firefox add-on that has been designed to provide users of Firefox for Android with means to run userscripts.
While Firefox desktop users have some choice when it comes to adding userscript support to their browser, Greasemonkey and the newly released Tampermonkey come to mind, none of these option appear to work on Firefox for Android.
The new Firefox add-on usi changes that by adding support for a wide variety of usrescripts. The author notes on the official add-on page that the add-on supports some Greasemonkey functions but not all of them.
What this means is that you may encounter userscripts that don't work yet when you install them with the help of the browser extension.
First thing you need to do is install usi. Visit the extension's AMO page and hit the "add to Firefox" button on it afterwards.
Confirm the dialog that appears to have Firefox install the add-on to make it available.
The easiest way to install userscripts afterwards is to load them directly in the browser. This is for instance the case when you visit Greasyfork, arguably the most popular script repository right now on the Web.
Simply select a script you like and click or tap on the "install this script" button afterwards. The script is loaded and its code is displayed on the page that opens.
The Firefox add-on picks it up automatically and displays an installation prompt to you. Confirm the prompt and the userscript gets installed on Firefox for Android.
You can do the same for local scripts that you want to add. Simply load them in Firefox and follow the installation instructions afterwards.
As noted earlier, you may run into scripts that won't work just yet. Another bug is that you cannot open the options directly to manage installed scripts. The add-on page on Mozilla AMO lists the resource URL that you need to open to access the preferences and management options.
There you find listed options to display all userscripts, create a new userscript, or load a userscript.
The options link provides you with three options right now:
The add-on is updated frequently, at least twice each month but usually more often than that. While it may take a while before it offers full userscript support (and then some), it is already working well in its current state.
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