[WIP] nav tweax flex

[x] https://bugzilla.mozilla.org/show_bug.cgi?id=1096654
[x] https://bugzilla.mozilla.org/show_bug.cgi?id=1083627
[x] https://bugzilla.mozilla.org/show_bug.cgi?id=1084016 (needs works at mobile width)
[ ] https://bugzilla.mozilla.org/show_bug.cgi?id=1059002 (scroll stuff needs work)

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# do not merge