Bug 1630234 - About:newtab does not maintain the scroll position after navigating back to the page - Notes
./mach mozregression –launch 2019-01-01 (check wether this has your bug or not)
if it did not, you can “mark” it as a good build : ./mach mozregression –good 2019-01-01 (again mark it as good)
it will then stop your browser, download a new snapshot, and you write wether its a good or bad build. Repeat until your window of when the bug appeared is so small its easy to find what revision it might have been introduced in.
Was using the Browser Toolbox to debug the problem