I downloaded and tested to see where that text color was at and it is part of the theme as you probably know.
You have three choices
#1
Ask the theme maker to change the menu text color.
#2
Find a custom theme for Firefox it will override the custom theme you are using in Windows.
#3
Purchase Window Style Builder and I could tell you where to change that text color at.
Jerry
My Computer
Computer type
Laptop
Computer Manufacturer/Model Number
Sager NP9170
OS
Win 7 Pro x64 / Win 10 Pro
CPU
Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
Motherboard
CLEVO P170EM
Memory
12 GB
Graphics Card(s)
(1) Intel(R) HD Graphics 4000 (2) NVIDIA GeForce GTX 675M
Sound Card
(1) Realtek High Definition Audio (2) Intel(R) Display Aud
Monitor(s) Displays
17.3
Screen Resolution
1920x1080
Hard Drives
(1) Samsung 860 EVO 500GB(OS) (1) Samsung 860 EVO 500GB(Data)
Keyboard
Backlite
Antivirus
MSE, Malwarebytes Pro
Browser
Firefox
Other Info
DESKTOP: Custom-built.
OS: Win 10 Pro x64.
CASE: Rosewill R5.
CPU: Intel I5 4670K CPU @ 3.40GHz.
CPU COOLER: Cool Master Hyper 212 EVO.
MOTHERBOARD: Asus Z87-A.
MEMORY: Kingston HyperX 2x4 GB.
GPU: Nvidia Geforce 650 TI.
PSU: Corsair TX750.
DRIVES: (1) Samsung 840 120 GB SSD (2) Western Digital blue 500 GB 7200 RPM.
There is no product of MS that will work on the .msstyles, the only way I know how to change the text color is with Ave's Windows7 Style Builder and that is what I prefer to use.
Jerry
My Computer
Computer type
Laptop
Computer Manufacturer/Model Number
Sager NP9170
OS
Win 7 Pro x64 / Win 10 Pro
CPU
Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
Motherboard
CLEVO P170EM
Memory
12 GB
Graphics Card(s)
(1) Intel(R) HD Graphics 4000 (2) NVIDIA GeForce GTX 675M
Sound Card
(1) Realtek High Definition Audio (2) Intel(R) Display Aud
Monitor(s) Displays
17.3
Screen Resolution
1920x1080
Hard Drives
(1) Samsung 860 EVO 500GB(OS) (1) Samsung 860 EVO 500GB(Data)
Keyboard
Backlite
Antivirus
MSE, Malwarebytes Pro
Browser
Firefox
Other Info
DESKTOP: Custom-built.
OS: Win 10 Pro x64.
CASE: Rosewill R5.
CPU: Intel I5 4670K CPU @ 3.40GHz.
CPU COOLER: Cool Master Hyper 212 EVO.
MOTHERBOARD: Asus Z87-A.
MEMORY: Kingston HyperX 2x4 GB.
GPU: Nvidia Geforce 650 TI.
PSU: Corsair TX750.
DRIVES: (1) Samsung 840 120 GB SSD (2) Western Digital blue 500 GB 7200 RPM.