Fix flex elements within <button> [M984689 and deps]

[classilla]

This is an issue on GoDaddy and Twitch, and probably many other sites given the number of duplicates.

https://bugzilla.mozilla.org/show_bug.cgi?id=984869
https://bugzilla.mozilla.org/show_bug.cgi?id=1308793
https://bugzilla.mozilla.org/show_bug.cgi?id=1316649

There is an assertion with https://bugzilla.mozilla.org/show_bug.cgi?id=1394631 but there doesn't seem to be a fix there. Shouldn't affect release; we could wallpaper if it does.

[classilla]

Doesn't even assert in debug.