Need some advice on KDE vs XFCE

[thenktor]

But what is not is that any attempt to install Firefox (3.6.13) brought the system to a total freeze. In case of FF from Salix repo - immediately at FF first start with Kwin crash, in case of Slack repo - slightly later, say, when adding some add-ons.

Which means in other words: The system is randomly freezing.
I can assure you that Firefox should not freeze the whole system in any case. The worst thing I've ever seen with Firefox is that it slowa down and renders incorrect because of wrong/missing graphics drivers.

[Shador]

Might be a kernel issue. In any case check dmesg, /var/log/syslog, /var/log/messages and possibly /var/log/Xorg.0.log.