Keyboard shortcut to move tabs around in Firefox on a Mac
There is no PgUp and PgDn on Mac keyboard but you can get them with fn+↑ and fn+↓ respectively.
So proper combo for moving is ⌃+⇧+fn+↑ (or in another direction last key should be ↓)
It's actually easier than it looks because first three keys are in left corner next to each other.