firefox.keepProfileChanges is ignored

with `firefox.keepProfileChanges` set to true, some of the browser preferences are not restored upon restart of the debugger.
The `prefs.js` file is updated and saved properly, but upon next start it's being overwritten by `user.js`

Start by reproducing the issue with firefox.keepProfileChanges set to true, then inspect how prefs.js is saved and how user.js is applied during the next debugger start. Done means preferences preserved by the debugger are not overwritten by user.js after restart.

Vom Indexierungsmodell aus dem Issue-Text verfasst.