> Debian testing.
I see that you use the latest version of the system,
maybe it could have some impact on performance.
> I wouldn't know since I use XFCE4 everywhere, but the desktop
> environment only has some impact. When it comes to Firefox, the main
> issue is Firefox itself and the pages you visit.
I had trouble with Firefox in the past on Windows 7,
I had to change to Chrome then. Surprisingly,
now Firefox is behaving relatively well.
I thought I would not have the same problem changing from Windows to Debian.