What do you think about implementing LibreJS as a server program and then having the extensions simply communicate with it?
Essentially, the browser sends the URL to LibreJS and waits for LibreJS to send a modified version of the page back, then renders that instead. Configuration would not be that hard - just install it via your distro's package manager and both the server and the browser extensions will be installed. During the first run, the extensions can ask for a valid host and port (which can be changed later), and other options (what to do when the server does not respond, for example). In terms of performance, I don't think this would be slower than a browser addon if the server runs locally: the work load would be moved to the back-end, which will be faster than the browser in any case. JavaScript on the JVM would be a good fit for it, I think. This fits with this ([1]) proposal and of course, making a LibreJS command line program (which RMS asked) would be trivial. Opinions? [1]: https://lists.gnu.org/archive/html/help-librejs/2015-04/msg00005.html
