Chances are that your taskbar is always cluttered and you hate it and you are helpless. MinimizeToTray Extension can help reducing Firefox and Thunderbird clutter but still, you might want to minimize other application to System Tray to reduce the clutter. There are some cool free application that allow you to do exactly this.
Windows users can save space on their Taskbar by downloading the nifty little program TrayIt. While viewing a maximized window, press CTRL as you minimize it on your taskbar. The program will no longer show up as a regular rectangle on the taskbar, rather, it will be a little icon in your tray. Just double click on the icon when you want to maximize the program again. Try TrayIt
4t Tray Minimizer minimizes programs to your system tray, much like TrayIt. 4t Tray Minimizer, however, has many other features - such as the ability to 'hide windows' and to launch applications on the desktop through a menu in the system tray. 4t Tray Minimizer adds a 'hide window' and 'minimize to tray' button next to the minimize, close and restore/maximize buttons. 'Hide window' hides the window, not sending it to the tray, and the only way to restore it is to launch the main window of 4t Tray Minimizer. Minimize to Tray, meanwhile, simply minimizes it to the system tray. 4t Tray Minimizer is Windows only, and is available for download as both freeware and commercial software.