Open a new tab and type "about:config" into the address bar. That should bring up a (large) list of all the browser options. Filter the options for the following: browser.cache.memory.capacity (create it as a new integer key by right clicking if the key does not exist already) and set the value to 16000. Which apparently limits the amount of memory that Firefox uses as cache to 16 mb. Another tweak to make Firefox more responsive is to change the config.trim_on_minimize value from true to false. This prevents Firefox from swapping itself out of main memory when minimized (with the subsequent delay when the browser is maximized).
And a couple of fantastic speedup tips for Firefox, outlined here. The pertinent tip is replicated below.
Find network.http.pipelining and double click on it so it is true
Find network.http.pipelining.maxrequests double click on it and change it from 4 to 100
A few more to do with how the pages are rendered by Firefox are also described here. In particular the keys to do with content.notify.* give the illusion that the page loads faster, by forcing Gecko (the page rendering engine) to display part of the page while it's waiting for the rest to arrive.
If you're using a fastish link, that's going to make pages load dizzyingly fast. Works for me.