Hi aeyoun,
thanks for this good question (and thanks for this great browser, it is still my favorite)
I would like to see two thinks on Mac which are probably already available on Windows:
Tab Cycling:
I like fast switching backward and forward between the last two tabs used by keyboard. This seems to be possibly currently only under Windows as mentioned in the flags. And there is also no extension available... please bring this back on Mac too
Remove Word form Dictionary:
In most dictionaries of any application s it is possible to add a unknown word, but none of there allow removing an accidentally added word, to prevent later mistakes by using it. The old Opera had this feature available and I used it often.... please bring it back.
Hilsen fra Tyskland
Achim