I don't think there is a way to do that. It does not seem like something they'd make configurable, if it was implemented.
It does seem like something they might want to implement, though. You can vote on it being implemented here (where someone else already submitted the idea): https://connect.mozilla.org/t5/ideas/drag-handle-on-firefox-browser-too-limited-add-ability-to-also/idi-p/104103
Side-note: In case you are on Linux, you may prefer the operating-system-wide shortcut to move any window from anywhere on its surface by holding either the Windows-key or Alt-key (varies between desktop environments) and then left-click-dragging on the window. If you do a right-click-drag instead, it resizes the window.