Hm, can't understand why this would really make a difference, in fact it should cause problems, as more memory would be needed as you would have to decompress the UPXed files. There would also be problems with updates, as Firefox would have to download full updates instead of patch files, if indeed it could still update itself. Much more useful would to minimise the number of addons you install, as well as watching which ones you use, as some add-ons slow down firefox more than others. This how-to only really applies to USB flash drives, which have very poor access times.

If you don't mind sacrificing a little memory, a program such as firefox preloader would do the job nicely. It loaded just the core of firefox into memory, without actually starting firefox, which is exactly what Internet Explorer does. However, it has not been updated in a few years, and only worked with Firefox 1.5.x. It could be made to work with Firefox 2.x by disabling the session restore functionality. I haven't tested it to see if it works with Firefox 3, as I'm happy enough with it's slow launch speed as I leave it running all the time.