I did a cursory search and did not see and extension there that will perform this function.
I suppose you want this for convenience's sake, as you don't want to have to minimize Firefox and then click the icon on your desktop?
If it is in your Quick launch bar you can easily access it there - but to have it show up as one of the 3 icons that always show (in the first 3 positions) you can always rename "Windows Mail" to "1 Windows Mail" and then sort by name - and it will put it in the first position.
Now, you will always see it in the task bar, and can easily use a single click to start it.
You can also use the shortcut [WINKEY] + [1] to start it since it is now going to be in the first position.
SanDisk Ultra SDSSDHII-960G-G25 960 GB SATA III SSD (System)
Crucial MX100 CT256MX100SSD1 256GB SATA III SSD (User Tree)
2 * Seagate Barracuda 7200.12 ST31000528AS 1TB 7200 RPM SATA II Mech. HD
Seagate ST1500DL001-9VT15L Barracuda 7200.12 1.5 TB S
PSU
Thermaltake Black Widow TX TR2 850W 80+ Bronze Semi-Mod ATX
Case
ThermalTake Level 10 GT (Black)
Cooling
Corsair H100 (CPU, dual 140 mm fans on radiator) + Air (2 *
Mouse
Logitech MX Master (shared)
Keyboard
Logitech G15 (gen 2)
Internet Speed
AT&T Lightspeed Gigabit duplex
Operating System
Sabayon Linux (current, weekly updates, 5.1.x kernel)
Manufacturer/Model
Lenovo ThinkPad E545
CPU
AMD A6-5350M APU
Motherboard
Lenovo
Memory
8 GB
Sound Card
Conextant 20671 SmartAudio HD
Monitor(s) Displays
Lenovo 15" Matte
Screen Resolution
1680 * 1050
Hard Drives
INTEL Cherryvill 520 Series SSDSC2CW180A 180 GB SSD
There's an old extension called 'External Application Buttons' I've been using since Firefox 0.8, needs bumping to work in FF3 but works fine and I see there's a ready-bumped version.
Any program you might want to start from within Firefox can be added, with its icon, to the Navigation toolbar.
SanDisk Ultra SDSSDHII-960G-G25 960 GB SATA III SSD (System)
Crucial MX100 CT256MX100SSD1 256GB SATA III SSD (User Tree)
2 * Seagate Barracuda 7200.12 ST31000528AS 1TB 7200 RPM SATA II Mech. HD
Seagate ST1500DL001-9VT15L Barracuda 7200.12 1.5 TB S
PSU
Thermaltake Black Widow TX TR2 850W 80+ Bronze Semi-Mod ATX
Case
ThermalTake Level 10 GT (Black)
Cooling
Corsair H100 (CPU, dual 140 mm fans on radiator) + Air (2 *
Mouse
Logitech MX Master (shared)
Keyboard
Logitech G15 (gen 2)
Internet Speed
AT&T Lightspeed Gigabit duplex
Operating System
Sabayon Linux (current, weekly updates, 5.1.x kernel)
Manufacturer/Model
Lenovo ThinkPad E545
CPU
AMD A6-5350M APU
Motherboard
Lenovo
Memory
8 GB
Sound Card
Conextant 20671 SmartAudio HD
Monitor(s) Displays
Lenovo 15" Matte
Screen Resolution
1680 * 1050
Hard Drives
INTEL Cherryvill 520 Series SSDSC2CW180A 180 GB SSD