For customizing Firefox to your personal needs and for maximum performance by using the options menue, close limits are set. But there is more:
By typing “about:config” into the address bar and hitting “Enter” you will access a vast list of parameters handling the behavior of your Fox. Using Firefox. 2.x you will access this list without a explanation about the risks, Firefox 3.x demands you to confirm, that you will be careful, once you done that, the fun may begin…

• Do not show the “Close Tab” Button on all open Tabs
  FF3 shows the “Close Tab” by default on every tab, if you do not want it to do so, change the config here:

  Parameter: browser.tabs.closeButtons
  Default value: 1
  Possible modifikations:

  • 0 [only shows the close button the active tab]
  • 1 [show it on all tabs]
  • 2 [do not show any close buttons on the tabs]
  • 3 [show only one closing button at the end of the tabs row]
• Adjusting the width of the tabs
  Firefox displays open tabs with a minimum width of 100 pixels, with many opened tabs the user will have to scroll the tab bar. To avoid scrolling or reduce it you can lower that minimal width.

  Parameter: browser.tabs.tabMinWidth
  Default value: 100
  Possible modifikations:
  0 [disables scrolling completely, any other values will determine a new minimal tab width]

• Turn off the session restore function
  Personally I do not like this feature very much, I cannot remember having used it willingly (sometimes I clicked the wrong button). So here’s how to turn it off:

  Parameter: browser.sessionstore.enabled
  Default value: true
  Possible modifikations: false [by setting the value to "false", the restore dialog won't appear anymore]

• Countdown timer before installing an Addon
  You’ve just found the Addon you were hoping for aeons of time, downloaded it, but then there are always these horrible two seconds of waiting till the installation begins…
  Get rid of them!

  Parameter: security.dialog_enable_delay
  Default value: 2000 [in milli seconds, which makes in this case 2 seconds]
  Possible modifikations: 0 [by setting the value to "0", the installation starts immediately, all other values, specify a delay in milli seconds]

• Turn off the Anitivrus Scan
  Only in Firefox 3: Downloaded files will have to pass an antivirus scan, which might take a while, if you are leeching a larger file.

  Parameter: browser.download.manager.scanWhenDone
  Default value: true
  Possible modifikations: false
  [by setting the value to "false", the scan is disabled, so watch out what you're downloading from which site]