I found this answer on another forum. I have at least a GB of ram in every machine I use and they're all recent so I never noticed this minimize problem. Maybe this will help you.
By default, when you minimize Firefox, it gives back to Windows most of the RAM the browser was using. It sometimes can take several seconds for the browser window to become fully loaded when you restore the window to its original size. This is one of those "version 1.0" things, in my opinion, that will become more streamlined in future bump revs of the program.
Fortunately, there's a hidden setting that can accomplish the restoration of Firefox windows much more quickly:
Step 1. Use about:config to create a new, Boolean value. Type or paste the following string into the dialog box that appears:
Step 2. Click OK to close the dialog box. Change the value from "true" to "false" and restart Firefox.
This doesn't piggishly retain all of the RAM that Firefox has claimed. If Windows needs more RAM to devote to another application, Windows can take it. What the setting does is prevent Firefox from giving up most of its RAM until the memory is actually required elsewhere.
Setting this item to "false" may noticeably slow down other applications while Firefox is minimized, if your PC has less than 256 MB of RAM. If so, change the setting back to "true."
Bumperzzz for all of the Firefox interest today!