If you must, must install a new Firefox externsion today, make it FasterFox (source).
What it does:
Prefetch Links
Dynamic speed increases can be obtained with Firefox’s unique prefetching mechanism, which recycles idle bandwidth by silently loading and caching all of the links on the page you are browsing.Tweak Network
Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, and initial paint delay.Block Popups
A popup blocker for popups initiated by Flash plug-ins is also included.Page Load Timer New!
A millisecond accurate page load timer tests the effectiveness of your settings.
After installation, and just keeping for the default configuration, I went to slowest-loading of all my regularly visited sites (I’m sadly on a dial-up connection only, you see): Flickr. Happy to say it did its job quite well, and I don’t see this externsion leaving my must-have Firefox list.
Now, if only there was an extension to alleviate Firefox’s pathetic use of system resources…