set browser.startup.blankWindow false

browser.startup.blankWindow = false - is default true on windows, not sure about other platforms

• see https://bugzilla.mozilla.org/show_bug.cgi?id=1448423
• nice to have for users who resize their browser .. oooooor we can wait for issues to arise in ESR140

also see #41592 (closed) @fkilic - see morgan's comments - so I hope the upstream patch is OK on linux cc @pierov