For versions of gretl that will run on older systems, see below.
If you have the rights of a "power-user" or better on Windows, choose a self-installer from the first or second column below; just download and run the file. If you have no administrator rights on Windows choose a zip archive from the third column; unzip this in any location where you have write permission.
If something doesn't work the way it's supposed to or you simply don't want to use GNOME 3.20 any more, you can purge the PPA and go back to the default GNOME version (3.18) available in Ubuntu GNOME 16.04.
You'll find instructions for how to do this at the end of the article.
Since the format of the cache files allows them to be mmap()ed shared between multiple applications, the overall memory consumption is reduced as well.
Is there stuff I should be doing to have GTK behave consistently?
For info, these are NOT system wide problems, and only seem to affect FF.
If you are running testing then you have probably had GTK3 updated to 3.20, which is not compatible with themes that have yet to be updated.
If necessary I will follow them but I'm a bit afraid of braking something.
NB: I use Debian-based Kali that's why the GTK appears so outdated.