I'm baffled as of why this happens, and I believe this might be gtk
related, however not all gtk3 programs I've tested do this so I'm
reporting this directly against firefox:
On x11, file->open (or ctrl+o), select any file, attempt to double-click
to open: nothing happens. File is is only selected. This happens
regadless if the file in the dialog has been selected previously or not.
Attempt to press enter: also nothing.
I have to explicitly click "Open".
Try the same with gimp which links to the same gtk library: both work as
expected.