If after having uninstalled mystartsearch, after having replaced it by google in your settings, in about:config, in regedit, after to have restarted your machine, you still have it, you can do the following :

C:\Users\Username\AppData\Roaming\Mozilla\Firefox\Profiles\profileid.default

Where bold parameters have to be replaced by the respective one.

Then open user.js with notepad and write https://www.google.com instead of mystartsearch url.

Now it finally works as in the past.