Thanks to bwat47 this is the fix that worked for the Intel HD Graphics 620 (Kaby Lake GT2) requiring only step 1 and the Iris Plus 640 (Kaby Lake GT3e) requiring step 1 and 2:

If any other information is needed let me know. Just did the most recent update and still seeing the tearing issue only in full screen on Firefox. I've primarily noticed it in Youtube videos the most.

Yes. Yes it is. Intel® HD Graphics 620 (Kaby Lake GT2) Intel Iris Plus Graphics 640 (Kaby Lake GT3e) both running current build of Solus 3.26.2 Full screen tearing in Firefox Quantum 58.0. Was able to "temporarily fix" the issue with compiz but compizconfig-setting-manager not working on 620 card. ccsm reports
Missing gtk2-engine-murrine and

• Error in `/usr/bin/python': free(): invalid pointer: 0x00007f12e6aad0d8 ***

I think I've pretty well narrowed this down to a CSS issue as it isn't displaying certain mouse pointers correctly. I know nothing about CSS maybe someone else can fix this?

I actually installed a fresh tarball of Firefox on live usb to test this on and found that I was still seeing the issue. Maybe it has something to do with the mouse theming in correlation to Firefox itself? Not sure...

GrisuProjects can confirm that the following cursor icons do NOT work with all updates applied while using the site you listed. Found this issue does not affect Google Chrome on my system. Everything works fine on Google Chrome. Only Firefox.

Just installed Deadbeef from software center. Tried to reproduce problem without success. Maybe another program is using a resource deadbeef needs?

Can confirm that switching from breeze-snow-curser to Adwaita (default theme) does not fix the issue. Also can confirm this happens while using the Solus live media as well.