If I have Firefox Nightly x64 opened and click on a URL link contained in a Thunderbird email, I receive a message warning that states that FF is running and need to close before I can open the selected URL link. See attached image.

I don't really want another FF session opened. What I would like is for the URL link I clicked on the email to open in a new tab (or on existing tab for all that matters).
Is that possible? Is there a setting I can toggle?
Thanks.

I don't really want another FF session opened. What I would like is for the URL link I clicked on the email to open in a new tab (or on existing tab for all that matters).
Is that possible? Is there a setting I can toggle?
Thanks.
My Computer
At a glance
Windows 7 Home Premium 64Bits SP1Intel i7-92012 Gb (6x2GB Corsair XMS TR3X3G1600C9 G) Tri ...ASUS GTX-970 Turbo
- Computer type
- PC/Desktop
- Computer Manufacturer/Model Number
- DIY Re-built.
- OS
- Windows 7 Home Premium 64Bits SP1
- CPU
- Intel i7-920
- Motherboard
- ASUS P6TD Deluxe
- Memory
- 12 Gb (6x2GB Corsair XMS TR3X3G1600C9 G) Tri Channels
- Graphics Card(s)
- ASUS GTX-970 Turbo
- Sound Card
- ASUS On Board SoundMax
- Monitor(s) Displays
- Samsung UN46D6500 46" TV
- Screen Resolution
- 1920x1080
- Hard Drives
- 1 OCZ SSD Agility Sata III 120Gb
1 Samsung 1Tb
1 Samsung 750Gb
1 Western Digital 500Gb(Mounted in ext eSata case)
- PSU
- Enermax CUG-950W
- Case
- Full ATX (Generic)
- Cooling
- Fans (6) 80mm
- Keyboard
- Saitek Eclipse II
- Mouse
- Cyborg R.A.T-5
- Internet Speed
- Comcast 105.0 MBPS
- Antivirus
- AVG
- Browser
- Waterfox v40.02(x64)
- Other Info
- - Logitech X-530 5.1 Sound speakers.
- APC ES-1300.
- Arris TG862G/CT Modem/Router.
- Samsung BD-083A Blue Ray Player
- Lite On iHBS212 Blue Ray Burner
- Nippon Labs All-In-One Card Reader/Writer W/USB & eSata
