Use mouse wheel to zoom in on images "inline" - that is - without a popup.
Hover on some image / video, press shift and use mouse wheel to zoom in and out.
Once you zoom in/out on an element, this element becomes draggable, useful in case the element is partially hidden (doesn't fit in its container).
The extension is listed on AMO - https://addons.mozilla.org/en-US/firefox/addon/in-zoom/
The extension does work fine in Chrome, but well... until it will be listed on Chrome Web Store the only way to install it is the manual one:
Download the .zip file here and unpack it
Go to the chrome://extensions URL
Check "developer mode"
Click the "LOAD UNPACKED" link and point chrome to the src subdirectory of the unpacked archive