Using Firefox 29 on Ubuntu 14.04. This also happened to me in FF 28 on 13.10. I have several tabs open, and when I switch to the tab I'm developing on, the inspect panel seems to randomly go blank. I'll right-click an element on the page and inspect it, the dev tools opens but the inspector area is empty. The CSS styles for the selected element appear as normal, however. I've found that if I drag the tab out of the window, to move it to its own new window, the developer tools disappears and when I re-open them, the inspect panel works again.