On KDE, long removable device label makes sidebar wider and prevents resizing
start KDE Plasma session
connect to your usb port a removable device labeled with a long name and mount it
open Firefox and load some webpage
press ctrl+s to open the filechooser
Now filechooser sidebar is wider because the long label of your removable device and it's impossible to resize it.
I recorded a screencast on Arch Linux running gtk3 3.22.29+4+gb485cf91-1 and Firefox 59.0.1.