-webkit-box doesn't work correctly in Firefox 112 and 113 so lets
default to flex again and make webkit-box opt-in. It is required for
Fx 114 though, but nightly users need to opt-in to using it.
This change makes compatible with pop-up windows. I use extensions that creates new pup-up windows and looks bad with that space in blank. this affects to other windows than don't use tabs.
Someone might use this style to make the bookmarks toolbar itself
vertical housing buttons etc. but have bookmarks-toolbar-items in some
other horizontal toolbar.