Create a userscript for the application.

[popman]

Currently I'm using the bookmarklet, but the time it takes to load the script and start running is irritating. (I often miss the start of the text)
This would also let you store visual and speed preferences on a browser level as well as set a hotkey to activate the script.

[peteruithoven]

We're talking about the following idea?
https://userscripts.org/about/installing

[popman]

I don't know if you mean something specifically by that page, but I am talking about the same kind of userscript.

[ds300]

I'd prefer to go the direction of a first-class firefox extension.

[popman]

It's not just firefox that would benefit from this, but if you wrapped the script into an extension (with something like this tool: https://arantius.com/misc/greasemonkey/script-compiler.php) it would at least be a stop-gap for a native extension.