I upgraded from FF 38 to 46 and found that the 38 about:config was over written and any changes that were made in 38 were lost; in particular I had set "browser.bookmarks.autoExportHTML" and "browser.bookmarks.file". Is this a bug or intentional? In either case, if the files in the new are different and new rules have been added, then the files should be merged, not overwritten.
They overwrite it every new update, I think. It particularly irks me that they keep enabling the spyware Hello.