I was being driven nuts by the same problem in LXDE and was able to resolve it by making the following change:
1. Enter "about:config" in the Firefox address bar.
2. Enter "browser.blink_allowed" in the search field.
3. Right click the entry, then select "Toggle" from the list of available actions.
4. This should change the value from "true" to "false".
Fair warning: I'm running a different distro (just happened to stumble upon this thread while searching for answers). It's still LXDE and Firefox, though, so I'm thinking it should work for you.