In Firefox 56, I think the images are drawn from this panel:

chrome://browser/skin/menuPanel.svg

Since they are SVG, it may be possible to apply a fill to them. However, they are applied as a list-style-image which I don't know how to color.

There must be a lot of dark theme style rules around the web that have solved this problem, so hopefully you can find a custom style rule that suits your needs.

Thanks for your help. I searched a lot, but I could not find themes that would change the color of the menu icon. How do I access and edit this file: chrome: //browser/skin/menuPanel.svg?

The SVG file is part of Firefox's program files and if you were going to modify it, you probably need to decompress omni.ja and change it in there. However, every Firefox update replaces that file, so a style rule solution would be preferable.

Also, Firefox 57 totally does away with the large icons on the application menu, so unless you plan to avoid updating, you might hold off and see what modifications you require after that.

I understand, So for a temporary fix, how can I extract the omni.ja file. When I try using jar -xf, I got an error.

Nill Morais said

I understand, So for a temporary fix, how can I extract the omni.ja file. When I try using jar -xf, I got an error.

This is beyond my expertise. Hopefully you can find the answer here or in a web search.

According to a lot of research, I figured out how to do it.

1. copy /usr/lib/firefox/browser/omni.ja to your ~/Desktop/.
2. rename it to omni.zip
3. unzip omni.zip -d omni
4. search for menuPanel.svg
5. use inkscape to select all, set white color and save
6. in terminal go to inside of omni folder
7. zip -qr9XD omni.ja *
8. sudo cp -f omni.ja /usr/lib/firefox/browser/omni.ja