I see this on latest FF 20, and maybe 've seen it before. It's seen often, for about 20% of resize operations.
When i do resize of FF window border, i.e. make window smaller, or move/drag window to the right, i see strange freezes of FF UI.
- FF can be closed during these freezes by Alt+F4, so not 100% freeze, it's semi-freeze
- Menu items don't drop down during these.
- Cursor is shown as "Resize arrow" during these.
- Freeze lasts for 5-20 sec, after that menu becomes active
I posted comment to FF developers (via Help menu), no reply. Win7 x64 Basic