One of the companies I work with has a web app hosted over a slow DSL line. There is something amiss with the configuration of their server or version of software they are using which means that every page access is triggering a download of a huge collection of JavaScript libraries. According to Firebug the actual contents of each page is delivered relatively quickly so.

Is there a way to force Firefox to consider a particular URL to have a longish expiry period?

Many years ago I used to do this sort of thing on squid proxy installs with custom regex rules to force a minimum cache age for misbehaving sites. What I'm trying to do seems similar but purely client side.