stop using XUL <grid> soon

Mozilla is removing support for XUL , probably in Firefox 78. References: https://bugzilla.mozilla.org/show_bug.cgi?id=1525737 https://groups.google.com/forum/#!topic/mozilla.dev.platform/yDiT7nEWe2c

Our Tor Browser patches use it here: https://gitweb.torproject.org/tor-browser.git/tree/browser/components/torpreferences/content/torPane.xul?h=tor-browser-68.7.0esr-9.5-1#n77

Tor Launcher uses it here: https://gitweb.torproject.org/tor-launcher.git/tree/src/chrome/content/network-settings-shared.js#n15