There are some tips about Optimizing Firefox V1.5+:
Firefox is supposed to dynamically realease memory from its RAM cache to other Windows applications as needed. Firefox power users recommend limiting the memory cache using the Configuration Console. This frees up memory for other apps, speeding up everything to a greater or a lesser extent, depending on
your machine and the applications you run. Here’s how the trick works:
Step 1. Type about:config into Firefox’s Address Bar and press Enter.
Step 2. Right-click any row, then click New, Integer. Type or paste the following preference name into the dialog box that appears (this is a hidden preference that doesn’t exist in the Configuration Console until you create it): browser. cache.memory.capacity
Step 3. Click OK, then enter the following integer number into the next dialog box, representing 16 MB of RAM for the cache: 16000
Step 4. Click OK to close the dialog box, then close all instances of
Firefox and restart it.

Move the disk cache to a faster or larger location:
Many Windows users like to locate their disk cachefiles on a separate hard drive from Windows. This can improve the performance of Windows or whatever program is using the cache, or it can free up space on a primary partition. (The disk cache should not be confused with the RAM cache described above.) To move Firefox’s disk cache, use about:config to create a new preference. (This is another hidden preference that doesn’t appear until you create it.)

Type browser.cache.disk.parent_directory into the dialog box that appears, then click OK. In the Value dialog box, enter c:\folder, providing the foldername in which you wish the file to be located.

Speeding up Firefox on Windows XP
You can speed up the loading of Firefox on Windows XP by adding it to the programs that XP “prefetches.”
To do this, right-click the Start Menu item for Firefox, or any icon you use to start Firefox, and open the Properties dialog box. Add a space plus /Prefetch:1 to the command line and click OK. The resulting line might look as follows:
“C:\Program Files\Mozilla Firefox\ firefox.exe” /Prefetch:1

Speeding up the maximizing of Firefox
Step 1. Use about:config to create a new, Boolean value. Type or paste the following string into the dialog box that appears: config.trim_on_minimize
Step 2. Click OK to close the dialog box. Change the value from “true” to “false” and restart Firefox.

Virtual Hard Drive takes a portion of your system memory (RAM) and uses it like a hard disk drive, dramatically increasing read and write times. Applications perform up to 340 time faster, making Virtual Hard Drive the software of choice for anyone looking to reduce the load and response time of calculation-intensive applications.
Creating a RAM disk on you hard disk, Moving Firefox’s cache directory there.
Have fun your Firefox life.