[Solved] Is it a bug of FireFox?

Question

You’re not using “CSS3”. You’re using a combination of invalid CSS (there is no plan for a display: box in CSS), an early WebKit draft implementation of CSS3 Flexbox, and XUL boxes (which are completely unrelated to CSS3 Flexbox and to what WebKit implements).

XUL boxes are not allowed to be floated; when you float them they become blocks, because floating changes display values in CSS.

Accepted Answer

You’re not using “CSS3”. You’re using a combination of invalid CSS (there is no plan for a display: box in CSS), an early WebKit draft implementation of CSS3 Flexbox, and XUL boxes (which are completely unrelated to CSS3 Flexbox and to what WebKit implements).

XUL boxes are not allowed to be floated; when you float them they become blocks, because floating changes display values in CSS.