IF the script doesn't use GM_ functions**, you can usually convert it to a bookmarklet and run it that way.
If the script uses the @require directive, then you must manually fetch the indicated file(s) and paste them into your bookmarklet.
Here is an online tool to help convert javascript code to bookmarklets.
You can also paste the javascript code into Firefox's console (CtrlShiftK), and run it that way. The same restrictions, as with bookmarklets, apply.
** But there is a limited-functionality shim that you can use to partially replace some GM_ functions.