As stated in the question.
As plenty of other posts here would have told you.
It would help to know what exactly you want to change..
Chrome has support for changing some of the settings, but via special APIs like contentSettings:https://developer.chrome.com/extensions/contentSettingsThere are more, just look at:https://developer.chrome.com/extensions/api_index
If these APIs are not supported by Firefox yet, maybe create a request on Bugzilla for it?https://bugzilla.mozilla.org/
Some specific preferences will be exposed to WebExtensions. There's a tracking bug for them here. There won't be open access to all preferences, however.