Firefox is GTK-based. With all its UI garbage like disappearing menus, CANCEL-OK dialogs (normal for Mac users), disrespect for system themes/styles/fonts, etc.
I also started using Falkon, but it still lacks too much for me, unfortuantely. And I wish it could import all history too. (
Lol. I've had dreadful feelings reading through comments - is windows explorer THAT BAD now? I'm still on 7 in native UI mode and blissfully unaware of terror MS have unleashed onto the world. And waiting for linux to become useful to switch.
I still use and can recommend Sequoia (or SequoiaView?) program from early 2000s to map all directories by size. VERY useful to know what to clean.
As for exlorer... The only drawback to your request is, obviously, the need for recursive FS scan, which is expensive. That is why it is not in-built in default file explorer functionality. But XnView, for example, optionally shows brief content of directories (e.g. 4 mini-thumbs over a directory icon).
Indeed, when I wrote a C utility and just wanted to output its start, finish and run time, I spent MORE TIME THAN IT TOOK TO WRITE THE WHOLE PROGRAM to figure out how the whole date/time garbage works! This was a painful, maddening experience. As if this entire API was designed to drive you mad.