Ask HN: Browser extension for limiting bandwidth (feature request)

I'm looking for a browser extension (ideally for Firefox, but Chrome could work too) that would limit the bandwidth to a user defined value. Especially for uploads, so that I could upload a file in the browser at, say, 50kB/s.

There's an old Firefox extension called "Firefox Throttle" that does this, but it only works with Firefox 3.6 (which is obsolete and doesn't work with many new sites).

I'm not a programmer, so excuse my ignorance, but in case something like this is easy to make, I'd be grateful and I know a few other people would appreciate it as well. I don't need a GUI, a setting in about:config would work just fine.

(Oh and I know there are some other solutions, but it needs to be a browser extension.)