In my experience on Mac the guilty party has consistently been the plugin container (or rather something inside it, presumably a bad video codec). `killall plugin-container` fixes the problem without having to restart Firefox, but unfortunately also crashes many if not most tabs (appears everything wants to use a multimedia plugin or another these days...)
So the "something inside it" could be script on a web page, or part of Gecko's rendering pipeline, or pretty much anything. And killig it crashes tabs because it's the thing rendering those tabs.
It might make sense to rename the executable to make things clearer, but there are some problems: there is Windows software that hardcodes the executable name and does things based on it, and changing the name would break various things for users....
On Mac and Linux, where this problem doesn't exist, the process naming is much saner...
Window transparency can be turned off by setting "gfx.compositor.glcontext.opaque" to true in about:config. This will cause a minor degradation in appearance of the window frame and tabs, but it will improve performance and extend battery life.
I have had it set for over 6 months and am anticipating the resolution of this outstanding bug.