I assume that it is for chrome. I have read that I can set the permissions in that file to avoid XSS errors while developing.
For the mac it's located here:
~/Library/Application Support/Google/Chrome/Default/Extensions
For further reading: Where does Chrome store userscripts.user.js files on a mac?
In linux mine are located in .config directory:
./.config/google-chrome/Default/Extensions/bjnkloegafmkhgpjglcbldhaokjpandj/1.0.0.0_0/manifest.json
./.config/google-chrome/Default/Extensions/gbammbheopgpmaagmckhpjbfgdfkpadb/1.9.1.8_0/manifest.json
./.config/google-chrome/Default/Extensions/aknpkdffaafgjchaibgeefbgmgeghloj/1.1.2.1_0/_locales/ar/messages.json
./.config/google-chrome/Default/Extensions/aknpkdffaafgjchaibgeefbgmgeghloj/1.1.2.1_0/_locales/zh-TW/messages.json
./.config/google-chrome/Default/Extensions/aknpkdffaafgjchaibgeefbgmgeghloj/1.1.2.1_0/_locales/es-419/messages.json
./.config/transmission/settings.json
./.config/transmission/stats.json
There are many json files for your chrome experience from Native chrome json files to json files for chrome extensions. Most of them can be found at
C:\Users\username\AppData\Local\Google\Chrome\
Most manifest.json files are found for extensions in
C:\Users\username\AppData\Local\Google\Chrome\User Data\Default\Extensions