How to merge settings between different firefox instances?

[rsbrux]

I have several different instances of Firefox running on different platforms under different user accounts.  I have shared my bookmarks among some of them by means of Firefox sync.  However, I have a big mess with the settings.  I am afraid to enable "settings" in Firefox sync, because some settings are platform-specific.  For example my download settings for Firefox snap under Ubuntu will not work in Firefox under Windows and vice-versa.  I would nonetheless like to profit from things I have configured in some instances (e.g. search engines and some settings in about:config) without having to "reinvent the wheel" on others. I need a way to selectively merge these settings.  IIRC, these settings are stored in JSON files.  Is there some way for me to selectively merge their contents without breaking Firefox?

[rsbrux]

I have managed to transfer search settings by simply copying search.json.mozlz4 from one profile to another (with Firefox closed, of course).  This is pretty safe, since the search engine parameters are platform-independent.  I am still looking for a way to merge other customized settings from about:config.  They are evidently all stored in prefs.js, which can be overridden by settings in user.js.  What I need is a tool to compare different copies of prefs.js to simplify extracting the important settings to a platform-independent user.js file.