Firefox Throttle worked for Firefox up to 3.6 alpha
"Firefox Throttle is a small plug-in for your favorite Web Browser, that allows you to cap download/upload rates and monitor current bandwidth utilization. "
Internet Explorer Throttle (As of 2010/02/23, still doesn't work for localhost)
"IE Throttle is a small plug-in for your Microsoft Internet Explorer, that allows you to cap download/upload rates."
Throttle seems to have been abandoned in that the links above take you to versions which only work with older browsers which are no longer available, so I have uploaded some files for archival purposes.
You can get Firefox 3.6.25 here, which still works with Throttle – or the binary installation file which I archived.
I like to use the Firebug Addon when testing with old slow browsers (part of the simulation). You can download it from my archive.
I made up a version of Throttle which will install and run with any Firefox above 1.4 and below 4.0 (as far as I know). It is just a simple hack on the max version check as described on other threads, but if you want an off-the-shelf Addon, my archive is
This does make a pretty nice compatability test for those of us who want to verify function on dial up lines, but don't have any dial up lines (and the older browser is consistent with who would be using dial up these days).